GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Need to know if your bino is working at its full and claimed aperture? Simple!
1) Bring your flashlight out of mothballs. The better type is one which emits near-to-white light (for brightness), and has a small emitting *area*. A slim, LED-based unit should be near ideal. But almost any 'normal' sized flashlight should work well enough. (My suggestion; the larger its reflector, the farther you should place it behind the eyepiece. This is to ensure that it is projected by the eyepiece onto the focal surface as a sufficiently small light source.)
2) Aim the light into the eyepiece, as close to on-axis as possible. A distance of 8-12 inches (20-30cm) behind the eye lens works well, unless your light has a really big reflector--in such case closer to two feet *might* be better.
3) Place a ruler a short distance in front of the objective.
4) Measure the diameter of the projected circle of light, as cast upon the ruler.
The principle is simple. If the eyepiece is set reasonably close to infinity focus, the beam of light emerging from the objective will be sufficiently close to parallel that its diameter will correspond to the *effective* aperture of the system. If the instrument's aperture is unobstructed, your measure will correspond to that 'advertised.' But *many* binos do not meet this criterion. Hence the need for a test such as this.
For example, if the front aperture of the prism cluster is too small to fully field the on-axis cone of light, the projected cylinder emerging from the objective will be proportionally smaller than nominal, indicating the *true* effective aperture of the bino.
A quick, simple and cost-effective test which I believe is warranted for nearly *all* binoculars. Unless, of course, you wish to be ostrich-like and stick your head figuratively into the sand.... 
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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94bamf
professor emeritus
Reged: 12/15/08
Posts: 706
Loc: Kansas City,Mo
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Interesting test you have there, Forgive me for asking, but what is the practical purpose of this test? I see alot of talk on these forums about true aperture, and binoculars being undersized. I am just wondering what action I should take if my binoculars fail to measure up? Should I quickly sell them, even though I might be happy with the views provided? I hope you will forgive me if I am sounding critical of your test here, because I certainly mean no offense to you for providing the method and information, it is much appreciated. It is just all the talk around here about true aperture and binoculars not measuring up, as if to say, somehow, they can no longer provide views that I might enjoy because they are only 44mm instead of 50mm. I guess this would be useful info for binoculars you are considering buying, if having full "advertised" aperture was really important to you. But if I already own the binoculars, it seems to me that all the results would do is depress me, or make me doubt the binoculars everytime I use them, always looking for a flaw.
I am certainly no expert on binoculars, I learn new things everyday just reading these forums. I think maybe sometimes we tend to overreact to the great information provided on this forum. How many people in the world could ever tell a difference with their eyes, between a binocular that has 44mm of true aperture vs the exact same binocular with 50mm of true aperture? It seems to me that Edz has great eyes, but has he ever documented a difference in his tests between two binoculars based on limiting magnitude or resolution, etc that could be attributed solely to reduced aperture, not some other variance in prisms, coatings, objective or eyepiece quality, etc?
Again Glen, I apologize if it seems I am attacking you. I just wanted to make a point about this subject, because it seems to be getting overblown. I find all the information provided here about different binoculars very useful. I certainly think knowing the true aperture of a pair of binoculars is interesting and useful. I would just suggest that the most important part, is the view through the binoculars.
I'll guess I will go stick my head back in the sand now.. 
Ken
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Telescopes:
Celestron C6 SCT on CG4 mount
Skywatcher 8 inch F/5 Newt on a GEM
Celestron 8 inch Starhopper Dob
Celestron Oynx 80ED
Celestron C130 Mak
Celestron C102HD
Binoculars:
Nikon 7x35 Action
Nikon 7x50 Action
Zen Ray Summit 10x42
Celestron 10x42 Noble
Orion 10x50 Scenix
Celestron 10x50 Noble
Pentax 12x50 PCF WP II
Celestron 15x70 Skymaster
Oberwerk 20x60
Zhumell 20x80
Edited by 94bamf (09/20/09 12:11 AM)
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Ken,
No need to apologize, and please pull your head out of the sand! I present this test for informational purposes only. If someone doubts reports on reduced effective aperture, it's so easy to verify. No need to rely on the word of the so-called 'experts.' And I might add that with a little understanding, the 'whys' of relative performance between binoculars will become apparent and appreciated. My only motivation is to educate.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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Glenn,
That is pretty smart, dude. Obviously, incorrect focus or a divergent flashlight beam will cause errors, in principle, which will be insignificant if the limiting aperture is at the objective lens, where the measurement is made. But, if the effective aperture stop is down deep in the binocular, it seems like those two errors could affect the result significantly. That is the very worst criticism that I can level at the moment, at your novel idea.
Ron
Ken,
You make a good point there. If you have a good binocular, it will show you a lot, and you tend to be satisifed with it. But, that is actually a bad attitude. Instead, you should always be highly annoyed at the possibility that somewhere there might be some so-and-so who's getting a better view than you.
I'd certainly rather have a good 44mm than a crummy 50mm. But a good 50mm will be 1.29x brighter, and that ain't hay. That is noticeable difference in number of stars, extension in nebula, and cluster cracking capability. My Fujinon 16x70 has enough spherical aberration at full aperture that too many stars look unsharp for the double-star lover in me, so I stop it down to 60mm much of the time. But if I am looking for dim stuff, either a particular object or summer Milky Way scanning, I remove the stops, and it really makes a difference.
Ron
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KBK
member
Reged: 08/05/09
Posts: 67
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The Kronos 26x70 gives a razor sharp (edged) 70mm-71mm with this done. I used a high output LED penlight.
The Yashica 7x50 gives a soft and one side squared (prism and innards) 50mm circle.
Circles measured as close to the lens as I could get and still read the rule.
Uniformity of light output across the circle (on a wall) of the Kronos was very high. I am in the projection business and it is one of my jobs to observe light uniformity, so I'm a little pickier than most when it comes to observing that aspect. Knowing good optical transmission when I see it is part and parcel of that job. I literally must be better at that than the many thousands who may use our products. It is only fair to note that this particular Kronos is modified for better contrast and/or control of the path.
However, the Yashica is done the same way, so the test as comparative pair, is a fairly decent test of the optics instead of that and inner light control, in this case involving my two sets.
What I am saying, is that the Kronos has a better organized lightpath with less internal scatter that the yashica cannot equal. If I put the circle of the emitted light on the wall (at close range), the Kronos shows no haloing or outside the circle scatter, while the yashica has some, besides the Yashica having an overall weaker observed pass-thru. High index vs low, is the case here, as well.
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It is the image of the ungraspable phantom of life; and this...is the key
to it all.
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94bamf
professor emeritus
Reged: 12/15/08
Posts: 706
Loc: Kansas City,Mo
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Quote:
My only motivation is to educate.
I certainly thank you for that.. 
I guess the only point I was really making is many people on the forums(myself included) sometimes take the great information provided here, and reduce it down to the simplest form (less than advertised aperture = BAD) without really understanding what REAL effect that might have on what an average human being can actually see through the binoculars. In other words, lets say I had a two binoculars that were exactly the same except one had 50mm of true aperture and the other had 44mm of true aperture. I hand them both to you or Edz, or 100 other people without telling you which is which. Could you or anybody tell which one was reduced by looking through them?
Sorry I am directing this at you Glenn..  
I just think these questions are important, so those of us that are interested, can gain a better understanding of the "real" effects of reduced aperture..
Ken
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Telescopes:
Celestron C6 SCT on CG4 mount
Skywatcher 8 inch F/5 Newt on a GEM
Celestron 8 inch Starhopper Dob
Celestron Oynx 80ED
Celestron C130 Mak
Celestron C102HD
Binoculars:
Nikon 7x35 Action
Nikon 7x50 Action
Zen Ray Summit 10x42
Celestron 10x42 Noble
Orion 10x50 Scenix
Celestron 10x50 Noble
Pentax 12x50 PCF WP II
Celestron 15x70 Skymaster
Oberwerk 20x60
Zhumell 20x80
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94bamf
professor emeritus
Reged: 12/15/08
Posts: 706
Loc: Kansas City,Mo
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Quote:
Glenn,
That is pretty smart, dude. Obviously, incorrect focus or a divergent flashlight beam will cause errors, in principle, which will be insignificant if the limiting aperture is at the objective lens, where the measurement is made. But, if the effective aperture stop is down deep in the binocular, it seems like those two errors could affect the result significantly. That is the very worst criticism that I can level at the moment, at your novel idea.
Ron
Ken,
You make a good point there. If you have a good binocular, it will show you a lot, and you tend to be satisifed with it. But, that is actually a bad attitude. Instead, you should always be highly annoyed at the possibility that somewhere there might be some so-and-so who's getting a better view than you.
I'd certainly rather have a good 44mm than a crummy 50mm. But a good 50mm will be 1.29x brighter, and that ain't hay. That is noticeable difference in number of stars, extension in nebula, and cluster cracking capability. My Fujinon 16x70 has enough spherical aberration at full aperture that too many stars look unsharp for the double-star lover in me, so I stop it down to 60mm much of the time. But if I am looking for dim stuff, either a particular object or summer Milky Way scanning, I remove the stops, and it really makes a difference.
Ron
Thanks for posting Ron. Do you think maybe that reducing the aperture at the actual objective might have a slightly different effect on the bino than it being reduced internally, like at the prisms, etc? I don't have all of Edz binocular reviews memorized, but it seems in atleast a few, even the binoculars that had the most aperture reduction had similar figures for limiting magnitude and other tests. Like in Edz 10x50 binocular test. Both the Pentax 10x50 and the Garrett 10x50 saw 9.8 magnitude stars, yet the Pentax(-3%) was one of the best about reduced aperture and the Garrett(-12%) was one of the worst. It seems as if there is more to their performance than just the useable aperture.
Ken
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Telescopes:
Celestron C6 SCT on CG4 mount
Skywatcher 8 inch F/5 Newt on a GEM
Celestron 8 inch Starhopper Dob
Celestron Oynx 80ED
Celestron C130 Mak
Celestron C102HD
Binoculars:
Nikon 7x35 Action
Nikon 7x50 Action
Zen Ray Summit 10x42
Celestron 10x42 Noble
Orion 10x50 Scenix
Celestron 10x50 Noble
Pentax 12x50 PCF WP II
Celestron 15x70 Skymaster
Oberwerk 20x60
Zhumell 20x80
Edited by 94bamf (09/20/09 01:38 AM)
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KBK
member
Reged: 08/05/09
Posts: 67
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As I quickly took note of, the test appears to be handy for checking on uniformity and scatter (on a wall), and this can be a notable part of the qualitative test or conclusion that we come to when setting eye to the things.
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It is the image of the ungraspable phantom of life; and this...is the key
to it all.
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Jokulainen
member
Reged: 08/10/09
Posts: 29
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Btw, is there some sort of list of true apertures in this forum?
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Man in a Tub
Not Retired!, But a little cranky!!!
Reged: 10/28/08
Posts: 2027
Loc: San Francisco, CA
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Quote:
Btw, is there some sort of list of true apertures in this forum?
Yes, for example, results for some selected binoculars (27) that Ed Z measured.
Small Binoculars - Exit Pupil & Aperture
Take note of the bold type that follows the second paragraph and precedes the initial results:
Quote:
This table has been completely revised, see the later post for the updated table with verified information.
Revised Table of Data
Download the attachment at that link. It also reports actual magnification measurements.
I found this about two weeks ago when researching the actual magnification and aperture of my Nikon 12x50 Action EX. Whew! It was gratifying. 12.2x49! That is very good. Now I know why I grab it so frequently now.
I tried Glen's test, but I measure a bit under Ed's result of 49mm averaged aperture. But since I'm a rank amateur with technicata (pardon my French!), I can live with what I measured tonight. And try again some other time with my other bins too.
Clear Skies!
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Todd
Brunton Eterna 15x51 ° Garrett Optical Signature Series 15x70
Nikon Action EX 12x50 ° Oberwerk 15x60 and 20x80 Standard
Orion Paragon Plus Mount and Paragon XHD Tripod
Garrett Optical Series 2000 Grip-Action Monopod
Edited by Man in a Tub (09/20/09 04:13 AM)
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zanti-misfit
member
Reged: 08/17/09
Posts: 88
Loc: SE United States
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If I'm reading the list right it seems that every Oberwerk tested was overstated by 7-8mm...if so that's disturbing.
I've been considering ordering one of their binocs too, the 12x60 or maybe the 11x56. So if this trend is brand wide that would look more like 12x52 or 11x49 right? If so I'm not digging that one bit at all. 
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Man in a Tub
Not Retired!, But a little cranky!!!
Reged: 10/28/08
Posts: 2027
Loc: San Francisco, CA
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Quote:
If I'm reading the list right it seems that every Oberwerk tested was overstated by 7-8mm...if so that's disturbing.
I've been considering ordering one of their binocs too, the 12x60 or maybe the 11x56. So if this trend is brand wide that would look more like 12x52 or 11x49 right? If so I'm not digging that one bit at all.
Probably. Ed likely didn't have an Oberwerk 12x60 or 15x60 to measure. I bought the Oberwerk 15x60 when I was in economy mode. I still use it, and I have no regrets. In the spring of 2008, the Oberwerk 15x60 gave me a boost for quick hand-held observing. I've rarely mounted it and have used and will continue to use it as a "finder" when I'm out with my larger GO SS 15x70 or Oberwerk 20x80 Standard. Right now, the infernal fog here has hampered more "systematic" comparisons of the Oberwerk 15x60 and the Nikon 12x50 Action EX. However, the magnification difference is notable and very real -- whatever the Oberwerk's actual magnification may be. So far, however, I'll say whatever I can see with the Oberwerk 15x60, I see with the Nikon 12x50 Action EX. Doubles or multiple star systems are another kettle of fish (?). Lousy metaphor.
Best Regards,
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Todd
Brunton Eterna 15x51 ° Garrett Optical Signature Series 15x70
Nikon Action EX 12x50 ° Oberwerk 15x60 and 20x80 Standard
Orion Paragon Plus Mount and Paragon XHD Tripod
Garrett Optical Series 2000 Grip-Action Monopod
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
if this trend is brand wide that would look more like 12x52 or 11x49 right?
Generally, issues like this are "model" specific, not brand specific. For instance, the Oberwerk Ultra model line does not suffer from reduced aperture.
In response to the "why do we even care question",
Limiting magnitude is not really the best indicator of "does reduced aperture matter". At the extreme low powers of binoculars, limiting magnitude is 3x to 4x more dependant on magnification than on aperture. Yes of course transmission comes into play, but it is surprising how little influence aperture may have. However, aperture really comes into its own when viewing faint diffuse extended objects. Not the Orion nebula, but perhaps the North America nebula or the Rosette, or IC342 or M74.
Consumer information is made available for the large numbers of people who like to know what it is they are buying before they buy it. It is no less valuable information for those that already own a particular model if comparing to some other model, however, the information is more useful to the consumer before they purchase.
There are a great many choices out there for the consumer. It is certainly better to be informed and make choices with that knowledge than to remain uninformed or ignore information and make choices without benefit. Likewise, for those that have more than one instrument on hand, it is more useful to be informed about which has the greater capabilities before selecting the tool and leaving with perhaps not another chance to change that selection for the duration of the observing session.
Rather know and be able to make a choice than not know and never be able to make an educated chioce at all.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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zanti-misfit
member
Reged: 08/17/09
Posts: 88
Loc: SE United States
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All of the Oberwerks you tested suffered badly. My guess would be that beyond their Ultra line the others would lose too. Sometime you'll have to try out the 60mm and 70mm ranges and see what develops. 
Thank you for the awesome info you've put together here at cloudynights. I've learned a lot from it. Being informed about stuff like this is very helpful, great job! 
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Jokulainen
member
Reged: 08/10/09
Posts: 29
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Quote:
edz
Hey btw, have you ever tested Celestron 15x70s?
EDIT: I tested it (or to be more precise, it's remnants) and it seems to be about 62mm..
Edited by Jokulainen (09/20/09 08:59 AM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Glenn,
That is pretty smart, dude. Obviously, incorrect focus or a divergent flashlight beam will cause errors, in principle, which will be insignificant if the limiting aperture is at the objective lens, where the measurement is made. But, if the effective aperture stop is down deep in the binocular, it seems like those two errors could affect the result significantly. That is the very worst criticism that I can level at the moment, at your novel idea.
Ron
Glenn, Ron
I tried this simple test this morning and found it to be quite easly performed and also quite accurate. I took out a 15x70 that I know is 63mm. I used a small bright LED white light. Actually I found there is quite a bit of leeway in the angle of the light entering and I could still easily see 63-64mm on a scale placed directly over the barrel.
One caution is the instruction to keep the light a reasonable distance back from the eyepieces. That is important. With the light right up near the eyepieces, a mistake someone might make is to tilt the light from side to side slightly. This allows you to shine the light right out to the objective edges on both sides, showing a false apparent aperture of 70mm. With the light perhaps 6" to 8" behind the objectives, I could tilt the light a little to each side and the effect was to "move" the 63mm projected circle across my scale.
BTW, the reduction in aperture in this binocular is a "too small" baffle opening, in front of the prism aperture, but still deep down inside the barrel and not near the objective. So it seems as long as the light source is a reasonable distance back from the eyepieces, a slight tilt will not affect the outcome of the test.
Of the several tests that have been documented to test aperture (briefly outlined below), this seems the simplest and most readily available to nearly every user.
Measuring exit pupil is magnification dependant, so requires two exacting tests, one (measuring magnification) which most users often fail to even recognize needs to be performed and furthermore most will never perform. The other, actually measuring the exit pupil, requires precise instrumentation and exacting placement, also which many users may not have or may not achieve. Measuring exit pupil is probably the least accurate way of predicting effective aperture.
Reading aperture through the eyepiece using a loupe requires the posession of a good loupe, a tool many people do not have, and care to not look around, but to look directly on-axis.
Using progressively smaller aperture masks and taking repeated readings of exit pupil will give an indication of both aperture and magnification, however, to get accurate results, requires accurate mask sizing, several masks, precise measuring of exit pupil and skill with mathematics, not so easy for everyone to achieve.
Measuring aperture using a target laser by projecting thru the aperture and out through the eyepieces requires a proper (newtonian collimation target type) laser, glass platten, and tripod. The fewest people would ever have the tools and patience to do this test.
This flashlight test is simple, straight forward and with reasonable care, accurate.
Thanks Glenn.
edz
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
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Positioning a small light source some distance from the EP (I like a foot or two) on axis will project the effective aperture on axis. Moving it off axis within the AFOV cone of the bin will project the effective aperture off axis and reveal vignetting. It's interesting to compare typical Porros to roofs. Stack them one on top of the other and move the light source around to see the relative off axis vignetting.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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RichD
professor emeritus
Reged: 11/08/07
Posts: 564
Loc: Derbyshire, UK
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That's a great little test!
I got the fuji 8x30 fmt as 30mm, the 16x70 as 69 or 70 and i'll have a go at my garrett 30x100 later.
Thanks Glenn
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Clear skies
Rich
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RobertPL
member
Reged: 03/04/08
Posts: 35
Loc: Austin, TX
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I suggest this thread be added to the 'Best Of' collection. It's that useful already.
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Fujinon 16x70 FMT-SX
Canon 10x30 IS
Orion MegaViews 30x80
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Man in a Tub
Not Retired!, But a little cranky!!!
Reged: 10/28/08
Posts: 2027
Loc: San Francisco, CA
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Can I nominate this thread for the "Best of" section? AFAIK, it doesn't duplicate anything there. It's a unique, practical and valuable contribution by Glen with additional and pertinent contributions from Ed.
Excuse me, I mean when this thread has "run its course" with comments, questions or whatever from others...
Best Regards,
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Todd
Brunton Eterna 15x51 ° Garrett Optical Signature Series 15x70
Nikon Action EX 12x50 ° Oberwerk 15x60 and 20x80 Standard
Orion Paragon Plus Mount and Paragon XHD Tripod
Garrett Optical Series 2000 Grip-Action Monopod
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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already planned to link it to "measurements"
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KBK
member
Reged: 08/05/09
Posts: 67
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Does anyone 'get' the points I raised? They are real and important.
The idea of getting a look at the emitted light on the wall at close range in a dark room, while making sure the light source used does not interfere in the test - will very quickly and accurately (but subjectively, of course-which is fine, that's how we use them) give one a sense of the issues of contrast in the given bino. The light direction/usage, with regard to human perception and orientation to the optics is the only change here.
If you try it with a set that is known by you to have low contrast and then compare to one that is known to have high contrast..well it should become relatively self evident at that point.
Attempts with Roof prism units, both coated and uncoated (phase), and then high and low quality should be interesting.
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It is the image of the ungraspable phantom of life; and this...is the key
to it all.
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milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
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Quote:
The idea of getting a look at the emitted light on the wall at close range in a dark room, while making sure the light source used does not interfere in the test - will very quickly and accurately (but subjectively, of course-which is fine, that's how we use them) give one a sense of the issues of contrast in the given bino.
Glenn's test is good for determining the true aperture but not contrast. The contrast of an optical system is measured by its Modulation Transfer Function (MTF):
http://www.astrosurf.com/legault/mtf.html
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Clear skies, Milt
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KennyJ
Reged: 04/27/03
Posts: 12889
Loc: Lancashire UK
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Glenn ,
Thanks for suggesting such a remarkably simple , accurate method of measuring effective aperture .
It came as no suprise for me to discover that all the models in my signature list seemed to deliver the full stated aperture with the exception of the only Chinese made model I own -- the 90mm Strathspey ( around 84mm ).
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
Interesting test you have there, Forgive me for asking, but what is the practical purpose of this test? I see alot of talk on these forums about true aperture, and binoculars being undersized. I am just wondering what action I should take if my binoculars fail to measure up?
It's useful but not the be-all and end-all.
To answer your question directly, if you have binoculars that are called 10x50, and you measure them and find they're 10x40, and you're also considering upgrading anyway, that measurement might provide an extra nudge.
But here are a few things to put this all in perspective. First, it's hard to measure to a millimeter or better, and the light source is *not* genuinely cylindrical. Between those two facts, expect an error of a few percent -- and don't get bent out of shape about it.
Second, a difference of 5% in aperture would probably be completely undetectable in practice, and would in any case matter less than other things that affect light throughput, such as the quality of the coatings.
Even 10% isn't vast. You can detect it comparing two instruments side by side, but it's fairly subtle. (I've done this with two identical telescopes, one masked, one not.)
When the error gets up to 15%, you have good grounds to get grumpy about false advertising.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Quote:
Interesting test you have there, Forgive me for asking, but what is the practical purpose of this test? I see alot of talk on these forums about true aperture, and binoculars being undersized. I am just wondering what action I should take if my binoculars fail to measure up?
It's useful but not the be-all and end-all.
To answer your question directly, if you have binoculars that are called 10x50, and you measure them and find they're 10x40, and you're also considering upgrading anyway, that measurement might provide an extra nudge.
But here are a few things to put this all in perspective. First, it's hard to measure to a millimeter or better, and the light source is *not* genuinely cylindrical. Between those two facts, expect an error of a few percent -- and don't get bent out of shape about it.
Second, a difference of 5% in aperture would probably be completely undetectable in practice, and would in any case matter less than other things that affect light throughput, such as the quality of the coatings.
Even 10% isn't vast. You can detect it comparing two instruments side by side, but it's fairly subtle. (I've done this with two identical telescopes, one masked, one not.)
When the error gets up to 15%, you have good grounds to get grumpy about false advertising.
Tony,
a difference in good coatings vs avg coatings might mean .25% to .5% per surface or 3% to 6% overall when compounded over 12 surfaces. A differe4nce in 5% of the diameter of the objective lens is a loss of 19% the light gathering area, 3x to 6x the effect of a difference in coatings. So a difference in aperure has far greater implications to total light.
And, no it is not hard to measure to a millimeter at all. I've prerformed three different tests on some binoculars and get all three results to agree within a millimeter.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KBK
member
Reged: 08/05/09
Posts: 67
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Quote:
Quote:
The idea of getting a look at the emitted light on the wall at close range in a dark room, while making sure the light source used does not interfere in the test - will very quickly and accurately (but subjectively, of course-which is fine, that's how we use them) give one a sense of the issues of contrast in the given bino.
Glenn's test is good for determining the true aperture but not contrast. The contrast of an optical system is measured by its Modulation Transfer Function (MTF):
http://www.astrosurf.com/legault/mtf.html
I did qualify the remark, and I did say it's utilization is a subjective comparative. I believe that in learned hands, it can show some important aspects.
Like a parrot being shown it's own image in a mirror, my dander got up big time (like it always does on this one), as soon as I encountered the word 'law' of diffraction at the supplied link. They mean 'Theory' of diffraction.
Laws are for civil considerations concerning humans who wish to push their overall agreed upon social parameters on the group as a control.
The word does not belong in physics -whatsoever. Common sense says it should be absolutely banished.
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It is the image of the ungraspable phantom of life; and this...is the key
to it all.
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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1393
Loc: Carson Valley, Nevada, USA
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I wish we could somehow go back in time and inform Kepler and Newton that their "laws" of mathematics and physics have now been demoted to mere "theory". I guess they should have consulted their lawyers first!
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
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charen
Pooh-Bah
Reged: 05/28/05
Posts: 1082
Loc: New Zealand
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Hi just measured some of my binos.
[Give or take 1 mm]
Oberwerk 100-45 degree B.T.s = 91 mm
Garrett IF 25x100 = 99 mm
Garrett IF 20x80 triplets = 75 mm
Oberwerk Ultra 15x70 = 70 mm
Nikon 10x70 HP IF WP = 70 mm
Pentax PCF WP11 20x60 = 55mm
Minox BD 10x58 ED BR = 60 mm ?
Oberwerk Ultra 10x50 = 50 mm
Nikon Monarch 10x56 roof = 55mm
B. and L. Discoverer 7x42 roof = 44 mm ?
The Oberwerk 100-45* B.T. at 91mm is a bit of a concern.
I measured it with the standard 25x E.P.s and several other 1.25 in. E.P.s and came up with the same measurement.
Can any one can confirm this with their 100-45* B.T.? do the E.P's themselves cause any vignetting ?
Chris
25 binos.
80mm Cat.
WO66ED
SV NH 80mm / EQ3
Meade 8in.LX90
Skywatcher Equinox ED120 / Goto HEQ5.
Edited by charen (09/21/09 02:57 AM)
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
And, no it is not hard to measure to a millimeter at all.
Fine, let me rephrase that. It's hard for *me* to measure to an accuracy of a millimeter -- and not for wanting of practice, either. And what's hard for me may be hard for others as well -- the audience to which Glenn aimed his post.
Note that with a conventional ruler (as opposed to calipers), you have to be very careful about parallax. Moving your head slightly will get you to a different millimeter tick.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Note that with a conventional ruler (as opposed to calipers), you have to be very careful about parallax. Moving your head slightly will get you to a different millimeter tick.
I don't see that happening at all. have a lot of leeway to move around to look at the ruler with absolutely no change in the reading.
From your comments it seems as though you haven't tried thius experiment. have you?
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
The Oberwerk 100-45* B.T. at 91mm is a bit of a concern.
I measured it with the standard 25x E.P.s and several other 1.25 in. E.P.s and came up with the same measurement.
Can any one can confirm this with their 100-45* B.T.? do the E.P's themselves cause any vignetting ?
I measured the straight thru Oberwerk BT100 at 92mm.
Eyepieces will have no effect on apparent aperture readings.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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RichD
professor emeritus
Reged: 11/08/07
Posts: 564
Loc: Derbyshire, UK
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That's a little surprising EdZ.
I get the Garrett 30x100 gemini WP at 99mm. That surprised me even more...
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Clear skies
Rich
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Rich V.
Pooh-Bah
Reged: 01/02/05
Posts: 1393
Loc: Carson Valley, Nevada, USA
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Miyauchi Saturn IIIs -- 100mm
Fujinon 16x70s -- 70mm
Pentax 10x50 PCF V -- 48mm
Pentax 10x43 DCF SP -- 41mm
Nikon 10x35 E2 -- 35mm
Nikon 7x35 E -- 35mm
Nikon 8x30 E2 -- 30mm
LOMO 80/480 triplet -- 80mm
Measured by projecting onto wall 6" away and using a digital caliper. Measurements were consistant with reading a scale right at the objective barrel but easier with the caliper.
Thanks, Glenn for this simple method of checking this!
Rich V
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Binoculars:
33/50/75/150x100 Saturn III, 16x70 FMT-SX, 10x50 PCF-V, 10x43 DCF-SP, 10x35 E2, 7x35 E, 8x30 E2, 7x26 Custom, 8x23AS Diplomat, 8x23 Travelite
Scopes:
C9.25, 6" f8 reflector, SV80S
Edited by Rich V. (09/21/09 01:17 PM)
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KennyJ
Reged: 04/27/03
Posts: 12889
Loc: Lancashire UK
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EdZ's comment about the different eyepiece focal lengths making no difference ought to be obvious , but just to amuse myself I tried looking at the illuminated circle with the zoom e.p of my Zeiss 20 - 60 at both extremeties , with no apparant alteration to the 85mm diameter of the objective .
The TeleVue also seems to be truly represented at around 75mm.
My 102mm Synta scope is out on permanent loan , so was wondering if anyone has checked the true aperture of any of these inexpensive chinese refractors ?
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1276
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The reservations I expressed earlier about this method are laid to rest. Thanks Glenn for the great idea, and to everybody who tried it out.
Ron
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milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
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Quote:
My 102mm Synta scope is out on permanent loan , so was wondering if anyone has checked the true aperture of any of these inexpensive chinese refractors ?
Kenny, I wouldn't worry about the Synta. If the objective measures 102mm and they didn't use an undersized baffle behind the objective - very unlikely - it would pass this test easily.
What surprised me was that your TV76 measured 75mm since TV doesn't even use conventional baffles. It could have been the measurement uncertainty that Tony mentioned.
Cheers,
Milt
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KennyJ
Reged: 04/27/03
Posts: 12889
Loc: Lancashire UK
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Hi Milt ,
With regard to the TeleVue , I did say " around " 75mm -- which is as near as I could see it on the ruler scale , and certainly close enough to 76mm for me !
It was certainly not intended as a criticism of the instrument -- far from it , and I could just as well have estimated 77mm I suppose ! :-)
Kenny
--------------------
If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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I'm glad to see the results posted so far. They do show a pretty clear trend for the higher end binos to work at least very close to full aperture.
Now to the [OPTIONAL] theory of the method...
This test was inspired as an alternate for a method I've used for years, whereby I simply looked into the objective toward an image point centrally placed in the eyepiece's image while moving my sight line laterally. This is OK for a quick 'n dirty check in the store, but it does require some degree of interpretation.
It recently occurred to me that a *tiny* light placed at the objective's focal surface for infinity, and located in the center of the eyepiece's field stop, would work similarly. With the added benefit of projecting a well-defined, *collimated* circle of light that's dead easy to measure the diameter of.
But putting a light *inside* the bino would require disassembly! How to get around that? Use the eyepiece as a projection lens and optically 'insert' the light inside.
As we all know, for an image point lying suitably distant from the observer, light from it arrives at the objectives following parallel paths. And these light rays emerge from the eyepieces (nominally) parallel, too.
Turn the instrument around and it works the very same way. Parallel light rays are brought to focus by the eyepiece at the field stop, and then emerge from the objectives parallel. But of course the entrant and exit pupils are reversed, and so the view is de-magnified.
By placing a light source reasonably distant behind the eyepiece, say 10 times the eyepiece focal length (typically 20-ish millimeters) or more, the light enters the eyepiece sufficiently parallel that it will come to focus quite near to that of the objective's infinity focus location. In this way an external light source placed 20cm or more from the eyepiece is projected as a rather smaller source at the coincident objective and eyepiece focal surfaces.
Using the smallest possible light works best, but the short focal length of the eyepiece ensures that the image is quite a bit smaller than the actual item. A smaller source will produce a sharper-edged emergent cylinder, which will provide a more accurate measurement.
Setting the eyepiece to the infinity focus position ensures that the emerging cylinder of light is collimated (parallel), thus eliminating error when the measurement must be made from a position fairly far from the objective.
Even if the light circle can be measured virtually in contact with the objective, a truly collimated beam is desirable. That's because a light-restricting baffle far down inside the instrument, e.g., a prism aperture for a long-barreled 100mm bino, determines the vertex angle of the cone of light arriving at the objective. If the image of the light source is too far in front of or behind the objective's infinity focal surface, the light cone's vertex angle will be too large or small, respectively. This will result in the effective aperture being measured as artifically too large or small, respectively.
The foregoing is probably of too small significance in reality. The focus range of an eyepiece is in all likelyhood just too short for this to be a source of worry. But it would be worthwhile to perform such an experiment just to see if a difference can be detected....
Parallax error during measurement will not be an issue when the circle of light is projected *onto* a ruler. Even a shiny steel rule will readily scatter enough light to render a well visible portion of the 'disk'.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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I do not have time just now to chase this topic, but are there not similar setups in the WW II Coleman report for Bu Ord, with his "Kinetic Definition" apparatus, or in Selby in his optical bench chapter in the ATM books, or in the Gaertner Optical Bench manuals? But they did not have LED flashlights or laser devices in days of yore. I have a complete Gaertner 1.5 meter optical bench, with numerous accessories, which has lain unused for years, should someone be interested in rigid setups for all kinds of measurements in either direction, at varying,precisely controlled angles, in well defined planes.
A question which is begging: Are the sub-advertised aperture effective aperture Chinese imports the result of incompetence,either by the designers or by the quality control people( are there some, at Barska, for example?), or of willfull deception, similar to the ruby colored coatings?
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Gordon,
I suspect the sub-aperture situation is quite deliberate. Surely, for an engineer at a large optical firm designing an instrument, it's the simplest thing to do a geometrical ray trace to check for these kinds of things.
I do believe that compromises are deliberately made when the overriding goal is to make an instrument small and light. That means using short focal length objectives to keep total length down, and smaller prisms to further reduce weight. The combination of steep light cone and eyepiece field stop being 'forced' just a little too far back (for lack of room beside the protruding adjacent Porro prism) to allow full illumination on axis.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 975
Loc: South of the Mason-Dixon Line
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Very interesting technique being discussed here. I took a sheet of paper and drew circles of various sizes to match my bino collection. The circles ranged from 25mm to 70mm. Most of my binos "passed the test" except for one standout: a Simmons 10x50 fixed focus model 24162. (Hey, I know fixed unfocused binos are a no-no but these were only $5 used from an astro meet swap shop and I figured on converting them into finderscopes.)
The Simmons showed a SQUARE of light from the Bk7 prisms. This square was smaller than it should be. If I imagined turning the square into a circle it would appear to be about 40-42mm.
Another bino that displayed a square was an old pair of Bell & Howell 8x40 porros that also has Bk7 prisms. However, the square of light appeared to be very close to a 40mm circle.
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Tools that make objects very far away appear much closer than they actually are.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Most of my binos "passed the test"
that's odd. I would have expected most of your binoculars to NOT pass the test, unless of course you have all top end brand/model binoculars.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 975
Loc: South of the Mason-Dixon Line
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Swift Audubon 8.5x44, Fujinon 14x70, B&H 8x40 and Burgess/TMB 12x42s were all as advertised. My Oberwerks 15x70 appear to be only 64mm clear aperture. A Burgess 10x25 Vireo is 2mm short.
My most often used and one of my most favorite bins is a Fujinon 2000 7x35 I have had for nearly 15 years. I had not checked it earlier as it was in another room in a bag with star charts, red flashlight, etc. I just got it out and checked it. Not only was light circle only 33mm or so the "circle" is obviously clipped by part of the prism assembly. None of the others showed this. Very disapointing.
So, including the Simmons 10x50 fixed focus model there were 3 out of 7 that "failed" and 4 that "passed". I need to amend my statement to "half" of them passed the test now including the 7x35 Fujis.
I'm still a little freaked over how the Bk7 prisms show a square.
--------------------
Tools that make objects very far away appear much closer than they actually are.
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
One of my most favorite bins is a Fujinon 2000 7x35 I have had for nearly 15 years. I had not checked it earlier as it was in another room in a bag with star charts, red flashlight, etc. I just got it out and checked it. Not only was light circle only 33mm or so the "circle" is obviously clipped by part of the prism assembly. None of the others showed this. Very disapointing.
I'll repeat what I said earlier. If these are favorite binos, it's for a good reason. The difference between 33 and 35 mm is trivial; it's almost inconceivable that you could detect it, even viewing through two instruments side by side. And some corners clipped by the prisms probably lose even less light-gathering area.
Sure, it's a little disappointing that a "premium" line doesn't quite meet it's specs. But in practical terms, it makes little or no difference.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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The superimposed square will be visible in the exit pupil also. The cause is the steeper rays from the objective edges striking the four reflective surfaces in the prism assembly at an angle too steep to allow total internal reflection. That's why BaK-4 is so commonly used nowadays; its higher index of refraction allows a steeper angle of incidence and still totally reflect.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 975
Loc: South of the Mason-Dixon Line
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Tony, you assuage me. I won't give up the 7x35 Fujis as there are too many good memories associated with it. Plus, it is light, small and has such a huge field.
BTW, if any of you guys have never had a chance to look at the Milky Way with one of those Swift Audobon 8.5x44s don't pass up the opportunity if it arrives. VERY immersive view with that combination of magnification and 70°+ afov. As of today I know it is a full 44mm aperture bino.
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Tools that make objects very far away appear much closer than they actually are.
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zanti-misfit
member
Reged: 08/17/09
Posts: 88
Loc: SE United States
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We tried this method of testing apertures on a few of our Bushnell binocs. The square was clearly there. Surprisingly, if we did it right, they all measured their advertised 50mm, and the other its 40mm. Measure only a couple of inches from the objective right? I held the binocs close to a wall and held the flashlight while somebody else measured the edges of the circle, even from one corner to the other of the square. Does it sound like I tested them correctly?
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94bamf
professor emeritus
Reged: 12/15/08
Posts: 706
Loc: Kansas City,Mo
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Quote:
BTW, if any of you guys have never had a chance to look at the Milky Way with one of those Swift Audobon 8.5x44s don't pass up the opportunity if it arrives. VERY immersive view with that combination of magnification and 70°+ afov. As of today I know it is a full 44mm aperture bino.
Could you give some more info on these binoculars? Are they still currently for sale new? I found THESE which only list around a 50 deg AFOV..
Thanks
Edit......Ok I found THESE too, these must be the ones? They list right at 70 deg AFOV. How is the edge performance?
Ken
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Telescopes:
Celestron C6 SCT on CG4 mount
Skywatcher 8 inch F/5 Newt on a GEM
Celestron 8 inch Starhopper Dob
Celestron Oynx 80ED
Celestron C130 Mak
Celestron C102HD
Binoculars:
Nikon 7x35 Action
Nikon 7x50 Action
Zen Ray Summit 10x42
Celestron 10x42 Noble
Orion 10x50 Scenix
Celestron 10x50 Noble
Pentax 12x50 PCF WP II
Celestron 15x70 Skymaster
Oberwerk 20x60
Zhumell 20x80
Edited by 94bamf (09/22/09 11:04 PM)
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Zanti,
Indeed, take your measurement from as close to the objective as comfortably possible. And to remind:
- Set the eyepiece focus reasonably near infinity.
- Keep the flashlight *at least* eight inches from the eyepiece (a foot or more is better, especially if the light is bright and/or strongly focused).
- Direct the light reasonably straight-on into the eyepiece.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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StarStuff1
scholastic sledgehammer
Reged: 04/01/07
Posts: 975
Loc: South of the Mason-Dixon Line
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Hi Ken,
The Swifts I have are the older No 804 model with 445 ft @ 1000 yds printed on them. This works out to about 8.5°. I have read that these were designed primarily for serious birders. The stars in my 804 start softening about 2/3 from the center to the edge and get worse. Still, the edges are better than many binos I have seen with smaller fovs. When you hold the Swifts up to the stars and take in the view by mostly looking at the center of the fov you don't notice the soft edge.
The bino the second link goes to is for a newer version with lighter weight and only a very slightly smaller fov. Being waterproof and rubber armored doesn't hurt, either. I can't really comment on the quality of view in this one since I have never looked through the 820 but I suspect the view would be very satifactory. The reviews I have read indicate that this is a very good bino for the money.
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Tools that make objects very far away appear much closer than they actually are.
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
Parallax error during measurement will not be an issue when the circle of light is projected *onto* a ruler.
Quite right -- I can't believe I didn't think of that (doh).
Since I do a lot of work on star charts and diagrams, I have a huge collection of rulers and other drafting tools -- all of them made out of clear plastic, which is *not* the right tool for this job. Using an old-fashioned wooden ruler, projecting the circle directly onto the ruler instead of onto a white surface behind the ruler, does indeed eliminate parallax.
I'm now sure that I can measure the aperture +-1 mm, which is ample accuracy in practice. Probably +-0.5 mm, though I'm not so sure about that.
It would be nice to have really high accuracy, because this exact same method can be used in reverse to measure the exit pupil. And given aperture and exit pupil, you can then measure magnification. (Average magnification over the entire FOV, anyway, which is *not* the same as magnification in the center of the field.)
Unfortunately, an 0.5-mm error measuring the exit pupil would give a 10% error measuring the magnification of binoculars with a 5-mm exit pupil. To be really useful, the exit pupil needs to be measured +-0.1 mm.
Although not nearly as accurate as measuring the aperture for an on-axis beam, this method also gives an excellent feel for how the beam is vignetted off-axis. Move the flashlight laterally, away from the central axis of the objective, then re-aim the beam at the center of the objective, so that it's illuminating the whole EP more or less evenly, but from an angle. You will see that the projected beam -- the effective aperture -- is no longer circular, once you get far enough off-axis. Other circles, and possibly the square prism edges, will start to clip the beam.
This isn't 100% accurate, for a variety of reasons. But it does hint at some purely geometric ways to measure the percentage illumination of the FOV once you get away from the center of the field -- the subject of an earlier note string.
Geometric methods can't take the place of measuring light throughput with a meter, because they ignore issues of light scatter from refractive surfaces, imperfect reflection from prisms, and absorption in the glass. However, geometric methods are *much* easier to apply than photometry!
More on this once I've done some experiments.
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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Andresin150
sage
Reged: 08/14/07
Posts: 305
Loc: Bogotá - La Calera / Colombia
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Not that I expected something different, but my giant Fujis measured 150mm exactly.... very easy to measure since you can project the beam in a paper placed over the dew shields, so it its perfectly aligned.
I'll post other "results" later
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25x150 MT
30-50x120's
Ultra 15x70's
GOTO NEX Planetarium
1 Macaw, 1 Toucan, 1 parrot and many other little ones...
And the nicest greenhouse!
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
It would be nice to have really high accuracy, because this exact same method can be used in reverse to measure the exit pupil. And given aperture and exit pupil, you can then measure magnification. (Average magnification over the entire FOV, anyway, which is *not* the same as magnification in the center of the field.)
Unfortunately, an 0.5-mm error measuring the exit pupil would give a 10% error measuring the magnification of binoculars with a 5-mm exit pupil. To be really useful, the exit pupil needs to be measured +-0.1 mm.
Well, basically that is the method I've been using for years to map exit pupil. The laser test.
This would only allow you to measure exit pupil IF you took the measurement at precisely the eye relief distance, which adds another measure of difficulty. As you move even 6" to 12" away, the circular projection grows to several inches large. Frankly, I wouldn't use this method for measuring exit pupil.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
This would only allow you to measure exit pupil IF you took the measurement at precisely the eye relief distance, which adds another measure of difficulty. As you move even 6" to 12" away, the circular projection grows to several inches large.
I don't see that. If you focus the device for infinity and throw a collimated beam into the objective, you should get a collimated beam out the eyepiece.
The key is that the light source has to be effectively at infinity in order to produce a collimated beam. But as with Glenn's test, you can get pretty close to a collimated beam simply by placing the light source far away -- around 10X the focal length of the lens you're illuminating.
An alternative, as Glenn suggests, is to use a telescope with aperture larger than the binocular to produce the collimated beam. In this case, you put the light source at the telescope's focal plane and the collimated beam shoots out the telescope's aperture.
Quote:
Frankly, I wouldn't use this method for measuring exit pupil.
Me neither, because I don't have the technology to measure the beam accurate to 0.1 mm.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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GlennLeDrew
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Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Following the slightly new direction of exit pupil measurement...
I measure exit pupil with a 10X loupe having a millimeter scale divided to 0.1mm. At 10X, the divisions are angularly so well separated that achieving 0.05mm is child's play.
While perhaps not quite significant enough to worry about in most cases, there is something one should be at least aware of. If the aperture-limiting stop is located relatively near to the eyepiece's field stop, its sharply-imaged edge will lie a small distance *behind* the image of the objective. This is because the eyepiece is a projection lens, and the closer to it an object lies, the farther back will the image formed by it lie.
You can verify this by simply looking into the exit pupil with a 7-10X loupe. Don't have one? Then an eyepiece of 20-30mm f.l. will do. You may have to turn it in reverse and/or remove its lower barrel, if focusing/viewing is difficult. As you slowly move the magnifier closer/farther, you'll see that you are successively bringing into focus various internal bits, such that inward you focus closer to the objective, and outward you focus closer to the prisms.
In the worst cases, where the offending stop lies *much* closer to the field stop than the objective, it's possible for the *new* exit pupil to lie far enough behind the image of the objective that the resulting slight increase in its magnification could become of some significance. But I'll have to do a more detailed analysis on this provisional theory!
At any rate, there *will* be a slight but artificial increase in the eye relief. My gut tells me that for a 20mm eyepiece this *could* amount to as much as a couple of millimeters. (While I highly doubt that this increased measure would be the reported value, I wouldn't discount the possibility entirely in some cases.)
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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Tony Flanders
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Quote:
Quote:
This would only allow you to measure exit pupil IF you took the measurement at precisely the eye relief distance, which adds another measure of difficulty. As you move even 6" to 12" away, the circular projection grows to several inches large.
I don't see that. If you focus the device for infinity and throw a collimated beam into the objective, you should get a collimated beam out the eyepiece.
Okay, tossing this over in the back of my mind at work, I saw a potential flaw in this reasoning -- which I verified by experiment when I got home.
What I said is technically true, of course. If you throw a collimated beam into the objective, you get a collimated beam out that eyepiece. That's the *definition* of infinity focus. But in real life, no beam of light can be perfectly collimated. What happens when the collimation is a little off?
When you shine a beam into the eyepiece of a telescope, the beam emerging from the objective has any miscollimation greatly reduced. But when you do it in the opposite direction, any error is greatly increased.
Let's try an example -- pretty close to what I just did on my kitchen table. Take a 10x50 monocular with an f/4 objective, focused at infinity. The objective's focal length is 200 mm, the eyepiece's focal length is 20 mm, and they're spaced 220 mm apart.
Now put the light source 3 meters from the objective. Instead of coming to focus at the proper spot, the beam comes to focus 1/(1/200 - 1/3000) ~= 214 mm from the objective. That's 6 mm from the eyepiece instead of the proper 20 mm, so the light beam coming out of the EP diverges strongly -- as though it were coming from a source 1(1/6 - 1/20) ~= 9 mm inside the instrument. Yow!
Now try this the other way. Shine the beam into the eyepiece from a distance of 15X the eyepiece's focal length, or 300 mm. The light comes to focus 1/(1/20-1/300) ~= 21.4 mm from the eyepiece, or 198.6 mm from the objective. So the beam coming out of the objective diverges as though it were coming from a source 1(1/198.6 - 1/200) = 28,000 mm away. For practical purposes, that's almost as good as infinty. No wonder Glenn's method is relatively insensitive to where you measure the beam!
In theory, you can eliminate the divergence by focusing on the light source -- at the cost of modifying the internal geometry of the instrument. But in practice, it's simply not possible to focus sufficiently accurately when shining the beam into the objective. I managed to reduce the size of the beam far from the EP quite a lot by careful focusing, but I never got it close to the size as measured at the exit pupil.
Moreover, even if you could focus perfectly, you'd still have to deal with the fact that the light source is not infinitessimal. But that's an entirely different kettle of fish.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Interesting analysis, Tony! And it nicely augments what I was cobbling up just now, probably while you were performing your experiment.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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zanti-misfit
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Reged: 08/17/09
Posts: 88
Loc: SE United States
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Thanks for the info Glenn. I'm going to do some more testing tomorrow and refine my process a tad.
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milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
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Quote:
So the beam coming out of the objective diverges as though it were coming from a source 1(1/198.6 - 1/200) = 28,000 mm away.
Tony, you have hit on the elegance of Glenn's method. I will try to put it even simpler for anyone still reading:
The definition of system magnification is the ratio of the angle that an off-axis ray exits the eyepiece to the angle that it entered the objective.
Going from objective to eyepiece the angle gets multiplied by the magnification, while going from eyepiece to objective the angle gets divided by the magnification.
The uncertainty in the number of off-axis rays picked up by the objective and passed through the field stop was what concerned me about Ed's illumination test.
Milt
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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"The uncertainty in the number of off-axis rays picked up by the objective and passed through the field stop was what concerned me about Ed's illumination test." Do you mean that the holographic target projecting laser does not have a uniformly illuminated pattern? Are you still "concerned"? Did I miss an applicable posting? The pattern is probably two lines, more or less interrupted, at right angles to each other, with a scattering of illuminated spots in the remainder of the circular pattern which is projected into the objective at an angle inclined to the optical axis, which is likely a compound angle ("skew"?) if the laser is handheld.
Is the concern about the blank portions of this projected, holographically generated, pattern? Would a solution be a widefield collimator, that is, a target collimator with a very large target, brilliantly and importantly, uniformly, illuminated,with an objective large enough to avoid vignetting within the collimator,and large enough to uniformly illuminate all parts of the binocular objective, shining into the objective of the binocular? The resulting target pattern, after leaving the eyepiece, would show any internal blockage, and the vignetting, on a larger scale than the flashlight test which works in the other direction.
Or, put the binocular on an astrophotography tracking mount, with a rotation support for the binocular to rotate around its yaw axis and use the(filtered?) projected solar image ( or defined part(s) of it(to avoid limb darkening)) to look for blockages and measure vignetting. One could also point the eyepiece at the sun and measure the effective aperture of the binocular with a ruler near the objective, ff the LeDrew test of this thread.
I just now did this reversed binocular solar test with a B&L MK21 WW II 7 x 50, freehand, and measure 50mm. A Swift Neptune Mark II 7 x 35, model 802, marked SK inside an equilateral triangle with truncated corners, gives 35mm. To my knowledge ,I do not have any dubious specimens likely to fail .
Another method, for quantitative vignetting measurement, might be to mount a small LED flashlight on an angularly adjustable base, shine it through the objective at defined and recorded angles, and use a photometer to record the throughput at the corresponding positions in the post-eyepiece pattern.
Has anyone dug out the NavOrd or BuOrd postwar report by Howard Coleman at Penn State? I could find my printed copies, but have not yet looked. I think that it may be online in some of the Abrahams material also. There maybe an applicable optical bench-like setup there.
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milt
professor emeritus
Reged: 09/13/04
Posts: 552
Loc: Arizona
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Quote:
Do you mean that the holographic target projecting laser does not have a uniformly illuminated pattern?....Did I miss an applicable posting?
Hi Gordon,
My concern was not for the laser test although I confess when did it I ignored the holographic portion of the image and only used the central beam. This is actually the only test I fully understand because it is a proxy for a single light ray entering one point on the objective at a defined angle.
The one that had me concerned was Ed's recent experiment using a non-coherent light source shining down a reflective tube into the objective and then measuring the total light output from the eyepiece. It seemed that all other things being equal, this would favor binoculars with larger field stops at the focal plane.
Quote:
Another method, for quantitative vignetting measurement, might be to mount a small LED flashlight on an angularly adjustable base, shine it through the objective at defined and recorded angles, and use a photometer to record the throughput at the corresponding positions in the post-eyepiece pattern.
I don't believe an LED flashlight is a coherent light source either...? In any case I think there is a place for both Glenn's light-in-the-eyepiece test to quickly measure true aperture and Ed's laser-in-the-objective test to ray-trace the system.
Milt
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aa5te
Genial Procrastinator
Reged: 08/30/08
Posts: 425
Loc: Clinton, TN
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I just did this test on a few of mine:
Tasco InFocus 10x50 = 39mm (ha ha! $10 K-mart clearance)
Pentax 20x60 PCF WP = 59mm
Apogee and Burgess Optical 25x100 = both around 95mm
Meade DS-2102AT 102mm refractor = 102mm (I did this one because the tube is only 90mm in diameter, but the lens cell is a few inches long (and wider), giving the light cone enough length to converge enough to not be clipped by the tube)
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Binos: Apogee 25x100 / Burgess Optical 25x100 / Pentax 20x60 PCF WP / Tasco InFocus 10x50
Tripods: Sunpak PlatinumPlus Ultra 7500TM / Quickset Samson
Refractors: Meade DS-2102AT-TC 102mm f/7.8; Sears Discoverer 3 (4454) 80mm f/15; Jason 313 Discoverer 60mm f/15.2
Reflectors (Dob): Hardin Optical DSH10 10" f/5
EP's: Zoom: Zhumell 8-24mm & Circle K 7.5-22.5mm 0.965"; 9-52mm Plössls; Zhumell 2x barlow
Kodak Z760; Orion SteadyPix camera mount
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Regarding that Tasco InFocus 'so-called' 10X50... Is the effective aperture the same for both halves? What's the exit pupil diameter? (This would verify the test, and furthermore would indicate whether the magnification is indeed 10X.) But no matter how you look at it, 39mm is one heck of a small effective aperture for a 50mm!
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
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KennyJ
Reged: 04/27/03
Posts: 12889
Loc: Lancashire UK
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Glenn ,
The so - called 10 x 50 Bresser Porros traditionally sold off in Christmas time sales at Lidl superstores here in the UK for £9.99 also only have a true aperture of around 40mm.
You may have seen the reference 10 x 50 ( 40 ) used a few times by various members here who own the model , which performs surprisingly well for astro use .
From my own non - technical tests when I had one ( I bought three to pass on to others ) I noticed the exit pupils were very close to 4mm in diameter and the magnification just a tad less than real 10x binoculars -- probably closer to 9.5x than 10x if expressed to the nearest 0.5x .
Indeed I once refered to the model here as 9.7 x 39 :-)
Kind Regards
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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aa5te
Genial Procrastinator
Reged: 08/30/08
Posts: 425
Loc: Clinton, TN
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39mm on both halves is verified! The exit pupil is right at 4mm, so the magnification would be 9.75x - close enough to 10x for government work. I guess this partially explains why I noticed such a large jump in brightness when comparing these to the Pentax pair - instead of the expected 10mm gain, there is actually a 20mm gain (not to mention superior quality, etc.).
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Binos: Apogee 25x100 / Burgess Optical 25x100 / Pentax 20x60 PCF WP / Tasco InFocus 10x50
Tripods: Sunpak PlatinumPlus Ultra 7500TM / Quickset Samson
Refractors: Meade DS-2102AT-TC 102mm f/7.8; Sears Discoverer 3 (4454) 80mm f/15; Jason 313 Discoverer 60mm f/15.2
Reflectors (Dob): Hardin Optical DSH10 10" f/5
EP's: Zoom: Zhumell 8-24mm & Circle K 7.5-22.5mm 0.965"; 9-52mm Plössls; Zhumell 2x barlow
Kodak Z760; Orion SteadyPix camera mount
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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I did the solar test on a Simmons "10x50" fixed focus Chinese "made in Hong Kong", says a label on the box. 39-40mm for both sides. I seem to recall that the Kamakuras had something to do with those. They were making many/all of the products sold by Bresser.
A Bushnell IF 8 x 40 wide field, probably from the 1950's, measures 40mm.
A Fuji Meibo 7 x 50 IF MTR ( from which I removed the deteriorated, sticky rubber covering) measured 50mm.
The solar test can be performed with two hands: one to hold the ruler, and one to hold the binocular. A piece of cardboard, plastic, wood, etc, with an opening through which the eyepieces can protrude toward the sun, will shade the ruler from direct sunlight, if that should be required.
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harbinjer
super member
Reged: 12/17/08
Posts: 126
Loc: Southeastern Minnesota
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It tried a Tasco Zip focus 10x50, and got about 44mm.
Also an old Yashica 7x50, which came up as 50mm.
The Audubon Raptor 8x42 seemed around 39mm.
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Rick
Post Laureate
Reged: 04/12/05
Posts: 3045
Loc: Tokyo, Japan
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After reading about the Canon 10x42L IS, I tested my 18x50IS and both objectives are perfectly circular 50mm with both exit pupils <= 3mm. Close enough for me!
thx Glenn!
Rick
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www.japanastro.com
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richtea
sage
Reged: 02/01/05
Posts: 241
Loc: UK Yorkshire
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Hi Glen and all
Great little test this even for non experts like myself
Tried on an old Swift 10 x 42 Ultralite porro and Bresser/Meade 10 x 50 budget porro last night
The Swifts were as close to 42mm as my eye could check and the Bresser/Meades were about 41mm as others have reported
Strange that so many beginners will purchase a binocular on manufacturers spec and/or advice on what spec is best when in fact the truth maybe quite different
Thanks
Regards
RichT
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Carl Zeiss MC Jenoptem 10 x 50 Nikon SE 10 x 42 Nikon E11 8 x 30/10 x 35 Meade 10 x 50 (40) Pentax Papilo 6.5 x 21 Minolta Activa WPFP 8 x 40 Praktica WA Lanthanum (x 2) 8 x 25 Swift Audubon 804 FMC 8.5 x 44 Fujinon CDPC Roof 7 x 42 Opticron Minerva Porro 9 x 35 Smith Wesson PC MC Roof 8 x 32 Bushnell Discoverer PC3XTR 8 x 42 Swift 10 x 42 Ultralite Hawke WP 10 x 42 porro
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BarrySimon615
Pooh-Bah
Reged: 03/01/04
Posts: 1279
Loc: New Orleans, LA
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I have tested some of my binoculars using the method discussed. I also had the advantage of having some disposable paper rulers used for measuring the length of newborns in the hospital. The rulers have both millimeter and inches. I simply taped a ruler to a door, set up a binocular on tripod and squared the binocular path to the door (leveled the optical axis of the binoculars and measured the distance of each side of the objective barrels to the door to make sure everything was aimed correctly.
Anyway here are my finding with perhaps the binocular that would be of most interest listed first -
Miyauchi BJ100 Galaxy 20x100 (and also measured with 26x eyepieces) - 98 mm, note the projected image of the objective was clean, sharp, evenly illuminated and did not show and chord cut off.
Celestron SkyMaster 25x100 - 92 mm, the projected image was edge fuzzy and also had the appearance of an eyeball with the eyelid partially down and covering the top of the image for both projected objectives. I played with this awhile thinking at first that it was my fault in how I was holding the light source, but I could not get this shading on the top half of both projected images to go away - hence very uneven illumination. When looking at the exit pupil, both looked round without any problems noted. Collimation is good, but the 92 mm aperture coupled with the very uneven illumination (eyelidding) is a concern. Maybe these binoculars would work for the Simpsons or Garfield the cat.
Pentax PCF Type V 16x60 - full 60 mm projected image, evenly illuminated, no issues
Nikon Action Extreme 10x50 - 49 mm, good projected image
Orion Savannah 10x50 - 49 mm, good projected image
Fujinon Series 2000 center focus 10x50 - 45 mm with chord cutoff
Carton Adlerblick 7x50 - 50 mm with clean projection and even illumination
Carton Adlerblick 8x42 - 42 mm with clean projection and even illumination
Oberwerk Mariner 8x40 - 32 mm
Fujinon Series 2000 7x35 - 33 mm with significant chord cutoff
I have a few more I could test but probably none of any real astronomical interest.
Barry Simon
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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My measurements agree with yours exactly or within 1mm on these
Celestron SkyMaster 25x100 - 91 mm
Pentax PCF Type V 16x60 - full 60 mm
Nikon Action Extreme 10x50 - 48 mm
Oberwerk Mariner 8x40 - 32 mm
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Patrik Iver
sage
Reged: 07/29/03
Posts: 259
Loc: Kaarina, Finland
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Checked the effective aperture of the binoculars that happened to be in the house just now:
Pentax Papillio 6.5x21 reverse porro: ~20 mm, some minor prism cut-off.
Leupold Yosemite 6x30 porro : ~28 mm, perfectly round.
Canon 10x30 IS porro-2: ~29-30 mm, perfectly round.
Minolta Activa 7x35 porro: ~33 mm, some prism cut-off.
Telescope Service (Kunming) Triplet 8x42 roof (not phase coated): Left barrel ~37-38 mm with largeish prism cut-off. Right barrel ~38 mm with less prism cut-off.
Zeiss Victory FL 7x42 roof: ~41-42 mm, perfectly round.
Kronos 26x70 porro: 70 mm, or maybe even very slightly larger, perfectly round.
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Best regards,
Patrik Iver
60°N, 22°E
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RichD
professor emeritus
Reged: 11/08/07
Posts: 564
Loc: Derbyshire, UK
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I see the uneven illumination on my Garrett 30x100 too Barry. Like you, I thought it was the way I was holding the light but it seems the problem is with the bino.
The eye analogy is a good one, and no matter how I present the light source to the eyepiece I can't get full illumination like I do with the Fuji 16x70.
Aperture however seems to be around 98mm, but I will recheck this given your result.
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Clear skies
Rich
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Patrik Iver
sage
Reged: 07/29/03
Posts: 259
Loc: Kaarina, Finland
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Quote:
Kronos 26x70 porro: 70 mm, or maybe even very slightly larger, perfectly round.
I just re-read this thread from the beginning, and noticed that KBK measured his Kronos 26x70 to be 70-71 mm, i.e. in accordance with my measurement.
I thought it a bit strange that they would appear to be slightly larger than specified, so I thought I would measure the objective lens diameter.
The trim rings around the ends of the barrels are on my sample between 68.86 and 68.95 mm (slighly out of round, measured with a digital caliper). I removed the trim rings and measured the objective retaining rings directly and found them to be between 69.98 and 70.16 mm (again slightly out of round).
The trim rings were in place when I measured the effective aperture using the flashlight method. Yet I see atleast 70 mm effective aperture with the trim ring forming a 69 mm aperture stop just a short distance from the objective lens. And KBK's observation seems to agree with mine.
I suppose the easiest explanation would be that I measured the effective aperture incorrectly (parallax or some such geometrical error)? I know the binos were set to infinity focus.
Could the fact that the light source is not an absolute point source cause a slightly too large reading which is more readily apparent in large aperture binoculars?
Any other ideas?
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Best regards,
Patrik Iver
60°N, 22°E
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
The trim rings around the ends of the barrels are on my sample between 68.86 and 68.95 mm (slighly out of round, measured with a digital caliper). I removed the trim rings and measured the objective retaining rings directly and found them to be between 69.98 and 70.16 mm (again slightly out of round).
The trim rings were in place when I measured the effective aperture using the flashlight method.
Well, it certainly cannot be any larger than the smallest aperture, which would be the trim rings.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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KennyJ
Reged: 04/27/03
Posts: 12889
Loc: Lancashire UK
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Patrik ,
It may be an interesting technical point as to why the measured circle of light appeared around 1mm larger in diameter than the actual measured aperture , but for practical influence and effect , I can't honestly imagine 1mm in 70mm making any significant difference .
Kenny
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Patrik Iver
sage
Reged: 07/29/03
Posts: 259
Loc: Kaarina, Finland
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Quote:
Well, it certainly cannot be any larger than the smallest aperture, which would be the trim rings.
edz
My thought exactly... Any thoughts on if the non point source nature of the light might cause this effect?
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Best regards,
Patrik Iver
60°N, 22°E
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Patrik Iver
sage
Reged: 07/29/03
Posts: 259
Loc: Kaarina, Finland
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Quote:
Patrik ,
It may be an interesting technical point as to why the measured circle of light appeared around 1mm larger in diameter than the actual measured aperture , but for practical influence and effect , I can't honestly imagine 1mm in 70mm making any significant difference .
Kenny
Agreed. I'm just curious in a purely academic sense...
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Best regards,
Patrik Iver
60°N, 22°E
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Quote:
Well, it certainly cannot be any larger than the smallest aperture, which would be the trim rings.
edz
My thought exactly... Any thoughts on if the non point source nature of the light might cause this effect?
I don't think so. I've done this simple measure now on a dozen or so binoculars using a high intensity flashlight. I come up within a mm of other methods of measure in every instance.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Patrik Iver
sage
Reged: 07/29/03
Posts: 259
Loc: Kaarina, Finland
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Quote:
Quote:
Quote:
Well, it certainly cannot be any larger than the smallest aperture, which would be the trim rings.
edz
My thought exactly... Any thoughts on if the non point source nature of the light might cause this effect?
I don't think so. I've done this simple measure now on a dozen or so binoculars using a high intensity flashlight. I come up within a mm of other methods of measure in every instance.
edz
Just now remeasured the aperture as carefully as I could. Setting up a target some 100 mm from the objective end and checking that everything was as perpendicular as I could get it. The result is ca 69 mm within a max error of maybe a couple of 10:ths of a mm. I.e. exactly as it should be considering the trim ring size.
So my original measurement must have had some geometrical error.
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Best regards,
Patrik Iver
60°N, 22°E
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
Could the fact that the light source is not an absolute point source cause a slightly too large reading which is more readily apparent in large aperture binoculars?
It not only can, but necessarily will. Whether the effect is big enough to account for your discrepancy is another matter. It depends largely on how far away from the objective you measure the outgoing light cone.
Think for a moment about what's going on. If the binoculars are focused for infinity, then a perfectly collimated cylinder of light entering the eyepiece will cause another perfectly collimated cylinder of light to exit the objective. If the outgoing light is a cylinder, then it doesn't matter where you measure it, and it's physically impossible for it to be bigger than the objective.
But in real life, this can never be realized, for three reasons. First, it's impossible to focus perfectly for infinity. Second, the light source isn't at infinity, so the light hitting the eyepiece is actually a diverging cone. Finally, the light source isn't a point, so the incoming light is actually a *converging* cone of *diverging* cones. (Draw a picture to understand what I mean -- I don't want to take the time now.)
All of these effects result in the outgoing light being a diverging cone -- and one with rather complex geometry. So the farther from the objective you measure it, the bigger it will be.
However, I still suspect that error of measurement dominates all of those effects combined. Honest, it's really pretty hard to measure a light cylinder or cone accurate to 1 mm.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Hi just measured some of my binos.
[Give or take 1 mm]
Garrett IF 25x100 = 99 mm
Garrett IF 20x80 triplets = 75 mm
Oberwerk Ultra 15x70 = 70 mm
...
Pentax PCF WP11 20x60 = 55mm
Chris
Hmmm, these should probably be verified. Iv'e tested all these same binoculars and in the same order, I come up with 92mm, 71mm, 69mm and 57mm.
Also some people reported on the GO 30x100. Well, its the same binocular as the 25x100, only different eyepieces, which has no affect.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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charen
Pooh-Bah
Reged: 05/28/05
Posts: 1082
Loc: New Zealand
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I will remeasure them more acurately over the next day or two using a 1 mm graph scale on the wall.
Chris
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35 binos.
80mm Cat.
WO66ED
SV NH 80mm / EQ3
Meade 8in.LX90
Skywatcher Equinox ED120 / Goto HEQ5.
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dave132mm
super member
Reged: 12/28/06
Posts: 125
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Nikon 12x56 DCF 56mm, very round, and even illumination.
Dave
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roadi
professor emeritus
Reged: 08/18/07
Posts: 531
Loc: Denmark
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Glenn, thats great info!! Thanks..
Now I've measured all my scopes, but I'm getting slight difrent results depending on EP used!
Homemade 150mm f5 binoscope: 24mm panoptics= 148mm +- 1mm ; 18mm LE = 148mm +- 1mm ; 12,5 Ortho = 145mm
Iluminated circle not as sharp defined as on my Sky90 Refr wich shows sharp 90mm ilumination with panoptic 24mm.
I recognised that my 12.5mm Ortho's has slightly difrent fieldstop!! one has 9mm and the other has 8.5mm!!
Polarex 60mm: R25 mm = 60mm sharp defined.
Mewlon 210: 24mm Panoptic = About 190mm Not sharp defined.
Tasco binocular 10x25 = 22mm.
A smaller iluminated diam circle than the aperture.. is that because of too small prism in binoculars?
Edited by roadi (10/29/09 07:59 AM)
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 505
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Just to report on this thread what was discussed on the Miyauchi Thread about a week ago.
GBT 100/45 true aperture - somewhere between 89mm to 95mm effective aperature. Defective tube 82mm to 87mm due to obstructions and 2" crack in tube wall.
BB
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Roadi,
When performing this test, a smaller-than-nominal illuminated circle for a bino does not necesarily imply a too-small prism. Most compact binos employ objective lenses having as short a focal length as can be used--sometimes *too* short. So it's the rather small *f/ratio* which leads to a reduction in effective aperture. As it is, most binos can just barely squeeze the steeply-converging *on-axis* light cone through the prism's front aperture. When this cone can't fit fully, we see a reduction in aperture.
Be aware that most hand held binos use objectives of f/ratio 3.3 to 3.8. If f/4 and longer were used, we pretty much never see these all-to-common smaller apertures.
Another thing to bear in mind. Small prisms in and of themselves are not responsible for aperture reduction. Here's why. As one goes to a larger prism assembly, the gain in aperture is exactly offset by the increase in optical path length through the now thicker glass.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Glenn, your explanation of the systems involved is absolutely correct, but, to say it's not the prisms that are too small, rather it is an objective with too short a focal length is sort of like saying, it's not this match that started the fire, it's those combustible materials.
If you assume for a moment that the full aperture and the eyepiece are matched to give, ie.25x100, then we must also assume the eyepiece focal length was chosen based on the apertures's focal length, so we could say these values are now fixed. Then if you don't get 100, it is a reasonable statement to say the prisms are not sized properly for the system.
I know, I know, an optics engineer can argue till we are all blue in the face that it is any number of things, and be correct, because it is. Well you can either start by holding something constant, (let's go with 25x100) and then stating what is not sized properly to match, or you can allow everything in the system to vary and claim any part as the culprit. If one were to say the 100mm lens is the culprit because the focal length is too fast, then we could change the focal length of the objective to see if it would work. We could come up with a focal length that would fit thru the prisms, but now we would then find that the binocular body and the eyepieces are wrong also.
BTW, in the smaller models in the Mariner/Gemini lines, it happens to be a grossly mis-sized baffle. In the larger ones there are no baffles.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Ed,
I stand by my assertion that it really is not an issue of prism size. But I'll have to qualify that, as such a bald statement assumes same-sized prisms and hence same-sized apertures at both the front and back of the assembly. For Porro systems, I call this a 'standard' arrangement, for lack of a better term. What follows immediately below pertains to this arrangement....
With a 'standard' prism, if one were to choose a larger porro pair, the gain in prism aperture would be exactly offset by the incresae in optical path length through the now-larger glass. In other words, a larger prism has its front aperture moved closer to the objective, where the converging light cone is now wider. The increase in prism aperture scales exactly with the increasing width of the light cone.
Where prism aperture *really* matters is at the back end, near the eyepiece's field stop. As long as the prism is not so small as to cause noticeable vignetting (darkening of the field edge), there is absolutely no reason to employ a larger prism set.
One could say that a bino is designed around its prism system. The prism's glass type dictates the shortest f/ratio for the objective. And the prism aperture imposes an upper limit to the field stop diameter if vignetting is to be avoided.
The magnification per se doesn't factor into this whatsoever, because irrespective of eyepiece focal length, the true field is determined by the field stop diameter. If the designer insists on lowewst magnification *and* a small prism, the AFoV will necessarily be quite small due to the long f.l. eyepiece being saddled with a tiny field stop.
=========
Now to the non-'standard' prism arrangement, one employing either a tapered or stepped prism. Such a prism has one half made wider so that its front aperture is larger than the others in the system. This allows to fully accommodate the steeply-converging light cone from an objective whose f/ratio would be a little too short for a 'standard' prism. (Such is the case for my Celestron 25X100, which employs approx. f/3.5 objectives.)
This is the only way to effectively increase prism aperture without suffering the counteracting increase in glass path length. But it's all too seldom seen in binos, unfortunately.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1767
Loc: Connecticut
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Too bad the tapered prisms aren't used more often. Is it as easy as using one large prism followed by a smaller prism? I would think this combination would be cheaper than two larger prisms. Or is there some other difficulty encountered by using different sized prisms?
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Peter,
I've occasionally wondered about using different-sized prisms, but I haven't yet examined in any detail what possible benefits such a combo might offer. It could well be a case of no real gain, as when up-sizing *both* prisms in a Porro. Or there could be a gain perhaps approaching 50% the efficacy of a tapered prism. Again, I'll have to look into it, especially now that my curiosity has been aroused.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1767
Loc: Connecticut
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Glenn,
I have just taken apart a 20x60 for cleaning and guess what I found? Large prisms mated to smaller prisms on each side. The large one is 1 1/16" wide and the small one is 3/4" wide. The shelf has 2 small apertures for the small prism and 1 large aperture for the entrance to the large prism.
I guess this isn't as unusual as I thought.
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 505
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Glen,
I would like your comments on info that I provided above and if I can add additional details, please let me know.
Was my comments overlooked/
Beachchairbill
Edited by beachchairbill (11/01/09 02:38 PM)
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dave132mm
super member
Reged: 12/28/06
Posts: 125
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Nikon 18x70 IF-WP 70mm very round, and even illumination.
Dave
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
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Did test, got about 93-95mm on my BT100s using 24mm Pans....got the world's heaviest pair of 95mm binos.
OTOH, circles were perfectly round and illumination appeared uniform across disc.
Don't understand why exit pupils with 24mm Pans measures precisely 4mm. If the aperture is undersized by baffle vignetting and focal length is 600mm as stated, exit pupil should be noticeably smaller.
Oh yeah, it looks like objective lens cell could be removed and rear baffle machined out larger to get full 100mm aperture, but they work so good now I don't know if I would want to mess with them. See Kevin's video on BigBinocular on "Why are the BTs so heavy?" to see sub components.
Edited by Mr. Bill (11/03/09 10:10 AM)
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
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Quote:
Glen,
I would like your comments on info that I provided above and if I can add additional details, please let me know.
Was my comments overlooked/
Beachchairbill
You mean regarding the effective aperture of the GBT100, and how it differs between the two tubes? I can't think of anything to ask....
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
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Maybe we should insist on binoculars being advertised stating effective aperture. I would say that the manufacturers are quite aware of what is going on (deliberately vignetting lightcone to clean up edge distortions and color)and in fact this is a form of deceptive advertising.
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
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Anybody measure the Garrett 20x110s or 28x110s?
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
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OK, did test from the aperture.....using toothpick positioned such that I could JUST see the toothpick point off axis with ep out (one advantage of removable eps)
Measurement was 3mm into lens edge which translates to 6mm diameter which matches my other measurement (93-95mm) using Glenn's method.
I think that the first baffle in is the bad boy....
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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Erik D
Post Laureate
Reged: 04/28/03
Posts: 3196
Loc: Central New Jersey, USA
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Quote:
Maybe we should insist on binoculars being advertised stating effective aperture.
I would like that. But,I am not sure how to go about listing the "EXACT" effective aperture unless we can first agree on a test methodology for the American Telescope Inspection & Testing Institute.
Let's take the BT 100 /45 deg and straight models discussed in this thread. I recall seeing figures ranging from ~ 91 mm to 95 mm depending on the configuration, EP and the person doing the measurement. Mr. Bill's own measurement indicates 93-95 mm. Would it be deceiving if the importer changes the figure to 94 mm but another member measures 91 mm?
We'll have to endow a fund to clone EdZ first.... ;-))
ERik D
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
Loc: Just passing through.....
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Notice signature change to reflect reality....
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
Loc: Just passing through.....
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I nominate edz for Binocular Czar....in charge of honest aperture numbers.
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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You know, actually we have documented in these forums at least five different methods of measuring the effective aperture. Only two methods have been proposed by me.
Measuring exit pupil has been around for decades (although first you must measure magnification to make that one reliable),; reading a scale by using a loupe over the eyepieces was proposed here I think originally by Henry Link (or perhaps Kimmo Absetz), and this easy method proposed here by Glenn (although we see care needs to be exercised for all three methods).
The other two methods are using a crosshairs target laser over the objectives as it is moved over a window, measuring extent across the window, and measuring changing exit pupil thru use of repetatively smaller objective masks (this has the advantage of also giving a close estimate of magnification).
The best advice is to confirm measurements obtained by one method with a result obtained by a second method. That's what I do. In fact, I compare results obtained by three methods, so there is really no question.
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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JCB
super member
Reged: 10/04/04
Posts: 197
Loc: France
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Quote:
Glenn,
I have just taken apart a 20x60 for cleaning and guess what I found? Large prisms mated to smaller prisms on each side. The large one is 1 1/16" wide and the small one is 3/4" wide. The shelf has 2 small apertures for the small prism and 1 large aperture for the entrance to the large prism.
I guess this isn't as unusual as I thought.
I have the same experience.
I have taken apart a Meade/Bresser 10x50 (20 euros Lidl binoculars), and to my surprise the prisms near the objective are 2 or 3 mm wider than the prisms near the eyepiece.
So it is probably a very common design. Naturally, that shortens the total optical path inside the prisms.
Jean-Charles
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
Loc: Just passing through.....
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Quote:
You know, actually we have documented in these forums at least five different methods of measuring the effective aperture. Only two methods have been proposed by me.
edz
Never thought much about differences in effective aperture and stated aperture until this thread. I always naively assumed that they were pretty much the same thing.
Frankly, I don't spend a lot of time on this stuff except when an obvious problem shows up.
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significent.
Also, the nagging feeling that I've been had. I hope my arriving (tomorrow) TMB is an honest 130mm clear aperture when you consider it costs (new) $800 an inch!!
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Never thought much about differences in effective aperture and stated aperture until this thread.
Hmmm. I've been publishing the effective aperture of binoculars that I've tested perhaps for about 5-6 years now. There are whole detailed threads, some with picture essays, and "Best Of" links pointing to all the methods of measuring effective aperture, including this one.
Perhaps you remember this thread from 2 years ago.
Small binoculars - Exit Pupil & Effectve Aperture
edz
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
Loc: Just passing through.....
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Quote:
Quote:
Never thought much about differences in effective aperture and stated aperture until this thread.
Hmmm. I've been publishing the effective aperture of binoculars that I've tested perhaps for about 5-6 years now. There are whole detailed threads, some with picture essays, and "Best Of" links pointing to all the methods of measuring effective aperture, including this one.
edz
Got lots of other stuff to read....
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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Tony Flanders
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Reged: 05/18/06
Posts: 3464
Loc: Cambridge, MA, USA
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Quote:
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significant.
Barely. Yes, you could detect the difference if you looked really carefully, but you'd never notice it if you weren't looking for it.
Here's my rule of thumb, based on experiments with masks:
- 5% increase in aperture -- barely noticeable, outweighed by considerations of absorption, scatter, etc.
- 10% increase -- readily visible comparing two otherwise identical instruments side by side
- 20% increase -- quite significant, qualitative change in views over a significant range of targets
- 50% increase -- very different, a whole new world
Still, significant or not, it's always reassuring when a vendor or manufacturer gives accurate specifications.
OTOH, vignetting from the focuser is a very common problem in small, fast refractors. In this case, the vignetting will vary depending on where the eyepiece's focal plane lies and how nearsighted the observer is. That makes it hard to give an accurate specification!
--------------------
Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3147
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Quote:
Quote:
A six percent difference in diameter works out to about 12% difference in light gathering which IMO, is significant.
Barely. Yes, you could detect the difference if you looked really carefully, but you'd never notice it if you weren't looking for it.
Here's my rule of thumb, based on experiments with masks:
- 5% increase in aperture -- barely noticeable, outweighed by considerations of absorption, scatter, etc.
- 10% increase -- readily visible comparing two otherwise identical instruments side by side
- 20% increase -- quite significant, qualitative change in views over a significant range of targets
- 50% increase -- very different, a whole new world
Still, significant or not, it's always reassuring when a vendor or manufacturer gives accurate specifications.
OTOH, vignetting from the focuser is a very common problem in small, fast refractors. In this case, the vignetting will vary depending on where the eyepiece's focal plane lies and how nearsighted the observer is. That makes it hard to give an accurate specification!
Caught that one on my Antares 6 inch f/6.5 (did thread and review of the problem) http://www.cloudynights.com/item.php?item_id=1274 Raytracing confirmed the problem; hacksaw solved it.
Still, I expected more out of a $1500 binocular sold by a reputable dealer.....lessons learned.
Every time I think I'm too cynical, reality wacks a 2x4 upside my head.
The Who...."Won't get fooled again" (and again and again, etc. etc.)
--------------------
10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT100 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS f/7 APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/8 homemade achromat
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 505
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GLD,
What do you think is my true apature for the GBT 100/45 with the info that I provided?
I know that Rich V. took a stab at it, however, he was hoping that either you or EDZ would add your comments as well.
Thanks,
Beachchairbill
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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BcB,
The figures you supplied covered a range of values--for each barrel, and I wasn't sure what was the source of variation. Did you indicate that for a particular barrel the entrance pupil is out-of-round? Other than that, I'd have to assume your measurements were as accurate as you could obtain, and have no reason to doubt them.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 505
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Glen,
The figures that I supplied were from the right barrel. This happens to be the undamaged barrel. Also, the out of round exists for both barrels and it only happens when you hold the flash light at 8" away and 16" away.
Beachchairbill
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1267
Loc: Ottawa, Ontario, Canada
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Celestron 25X100, purchased in 2004: 100mm! The circle is perfectly round and sharp-edged.
This photo shows why.
In a nutshell, it's because of the stepped prism which first fields the light cone from the fast, approx. f/3.5 objective.
--------------------
Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
|
beachchairbill
professor emeritus
Reged: 11/06/08
Posts: 505
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Could it be that the 25x100 are fixed ep's and the GBT's are interchangeable?
BB
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14724
Loc: Cumberland, R I , USA42N71.4W
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|
Quote:
Celestron 25X100, purchased in 2004: 100mm! The circle is perfectly round and sharp-edged.
This photo shows why.
In a nutshell, it's because of the stepped prism which first fields the light cone from the fast, approx. f/3.5 objective.
Interesting that yours are full aperture. Two others of us recorded the Celeston Skymasters at 90-92mm aperture. I wonder if they changed somewhere along the line?
edz
(I see you have the same question in your mind as well)
--------------------
Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
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