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True aperture? A Quick Way to Measure!

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#76 EdZ

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Posted 23 October 2009 - 01:08 PM

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

#77 KennyJ

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Posted 23 October 2009 - 01:11 PM

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

#78 Patrik Iver

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Posted 23 October 2009 - 01:12 PM

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?

#79 Patrik Iver

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Posted 23 October 2009 - 01:15 PM

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... :)

#80 EdZ

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Posted 23 October 2009 - 01:22 PM

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

#81 Patrik Iver

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Posted 23 October 2009 - 01:58 PM

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.

#82 Tony Flanders

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Posted 24 October 2009 - 05:00 AM

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.

#83 EdZ

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Posted 24 October 2009 - 11:01 AM

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

#84 chris charen

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Posted 24 October 2009 - 01:34 PM

I will remeasure them more acurately over the next day or two using a 1 mm graph scale on the wall.

Chris

#85 dave132mm

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Posted 25 October 2009 - 12:21 PM

Nikon 12x56 DCF 56mm, very round, and even illumination.


Dave

#86 roadi

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Posted 29 October 2009 - 06:53 AM

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?

#87 beachchairbill

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Posted 29 October 2009 - 05:20 PM

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

#88 GlennLeDrew

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Posted 29 October 2009 - 05:35 PM

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.

#89 EdZ

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Posted 30 October 2009 - 07:02 AM

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

#90 GlennLeDrew

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Posted 30 October 2009 - 06:44 PM

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.

#91 pcad

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Posted 30 October 2009 - 07:12 PM

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?

#92 GlennLeDrew

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Posted 30 October 2009 - 08:55 PM

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.

#93 pcad

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Posted 31 October 2009 - 10:42 AM

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.

#94 beachchairbill

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Posted 31 October 2009 - 10:04 PM

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

#95 dave132mm

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Posted 01 November 2009 - 07:15 AM

Nikon 18x70 IF-WP 70mm very round, and even illumination.

Dave

#96 Mr. Bill

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Posted 02 November 2009 - 07:38 PM

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.

:question:

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.

#97 GlennLeDrew

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Posted 02 November 2009 - 08:25 PM

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....

#98 Mr. Bill

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Posted 03 November 2009 - 10:07 AM

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.

#99 Mr. Bill

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Posted 03 November 2009 - 10:13 AM

Anybody measure the Garrett 20x110s or 28x110s?

:question:

#100 Mr. Bill

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Posted 03 November 2009 - 11:02 AM

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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  • 3427195-Vignette test.JPG



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