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True aperture? A Quick Way to Measure!

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#1 GlennLeDrew

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Posted 19 September 2009 - 09:22 PM

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.... ;)

#2 94bamf

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Posted 19 September 2009 - 11:08 PM

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.. :grin:

Ken

#3 GlennLeDrew

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Posted 19 September 2009 - 11:27 PM

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.

#4 ronharper

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Posted 19 September 2009 - 11:48 PM

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

#5 KBK

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Posted 20 September 2009 - 12:04 AM

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.

#6 94bamf

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Posted 20 September 2009 - 12:07 AM

My only motivation is to educate.


I certainly thank you for that.. :bow:

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.. :lol: :help:

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

#7 94bamf

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Posted 20 September 2009 - 12:33 AM

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

#8 KBK

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Posted 20 September 2009 - 12:45 AM

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.

#9 Jokulainen

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Posted 20 September 2009 - 01:02 AM

Btw, is there some sort of list of true apertures in this forum?

#10 Man in a Tub

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Posted 20 September 2009 - 03:10 AM

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:

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!

#11 zanti-misfit

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Posted 20 September 2009 - 03:22 AM

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. :confused:

#12 Man in a Tub

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Posted 20 September 2009 - 04:04 AM

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. :confused:


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,

#13 EdZ

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Posted 20 September 2009 - 05:16 AM

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

#14 zanti-misfit

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Posted 20 September 2009 - 05:35 AM

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! :jump:

#15 Jokulainen

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Posted 20 September 2009 - 05:36 AM

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..

#16 EdZ

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Posted 20 September 2009 - 07:09 AM

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

#17 Luigi

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Posted 20 September 2009 - 07:52 AM

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.

#18 RichD

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Posted 20 September 2009 - 08:03 AM

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

#19 RobertPL

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Posted 20 September 2009 - 08:26 AM

I suggest this thread be added to the 'Best Of' collection. It's that useful already.

#20 Man in a Tub

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Posted 20 September 2009 - 08:37 AM

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,

#21 EdZ

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Posted 20 September 2009 - 08:58 AM

already planned to link it to "measurements"

edz

#22 KBK

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Posted 20 September 2009 - 10:52 AM

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.

#23 milt

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Posted 20 September 2009 - 11:46 AM

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...egault/mtf.html

#24 KennyJ

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Posted 20 September 2009 - 12:42 PM

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

#25 Tony Flanders

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Posted 20 September 2009 - 04:29 PM

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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