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True aperture? A Quick Way to Measure!

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#26 EdZ

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Posted 20 September 2009 - 04:51 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.


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

#27 KBK

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Posted 20 September 2009 - 04:56 PM

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


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.

#28 Rich V.

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Posted 20 September 2009 - 06:39 PM

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

#29 chris charen

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Posted 21 September 2009 - 01:18 AM

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.





#30 Tony Flanders

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Posted 21 September 2009 - 05:03 AM

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.

#31 EdZ

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Posted 21 September 2009 - 05:57 AM

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

#32 EdZ

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Posted 21 September 2009 - 05:59 AM

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

#33 RichD

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Posted 21 September 2009 - 07:26 AM

That's a little surprising EdZ.

I get the Garrett 30x100 gemini WP at 99mm. That surprised me even more...

#34 Rich V.

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Posted 21 September 2009 - 08:26 AM

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

#35 KennyJ

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Posted 21 September 2009 - 11:13 AM

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

#36 ronharper

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Posted 21 September 2009 - 01:14 PM

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

#37 milt

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Posted 21 September 2009 - 02:52 PM

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

#38 KennyJ

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Posted 21 September 2009 - 03:00 PM

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

#39 GlennLeDrew

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Posted 21 September 2009 - 06:04 PM

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'.

#40 Gordon Rayner

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Posted 21 September 2009 - 06:15 PM

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?

#41 GlennLeDrew

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Posted 21 September 2009 - 06:32 PM

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.

#42 StarStuff1

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Posted 22 September 2009 - 11:14 AM

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.

#43 EdZ

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Posted 22 September 2009 - 03:28 PM

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

#44 StarStuff1

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Posted 22 September 2009 - 04:49 PM

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.

#45 Tony Flanders

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Posted 22 September 2009 - 06:14 PM

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.

#46 GlennLeDrew

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Posted 22 September 2009 - 06:17 PM

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.

#47 StarStuff1

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Posted 22 September 2009 - 09:38 PM

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.

#48 zanti-misfit

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Posted 22 September 2009 - 09:59 PM

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?

#49 94bamf

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Posted 22 September 2009 - 10:01 PM

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

#50 GlennLeDrew

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Posted 22 September 2009 - 10:39 PM

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.


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