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Anonymous
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I'm trying to measure the size of the exit pupils of my binoculars, but am having a hard time. I started by trying to project to a wall and measure the distance to that wall and the diameter of the circle created, but soon found a better way.
I took a cardboard cylinder and attached it to my eyepiece. If I measure from the very edge of the lens to the place on the inside of the tube where the light of the exit pupil first strikes it, I will know how long it takes for the exit pupil to spread out to the same size as the diameter of the cylinder. But now I have no idea how to figure out the actual size of the exit pupil. I know all about light spreading out to cover four times the area as the distance is doubled, but I don't know were it starts.
If the light simply spreads out right from the edge of the exit pupil all I would need to know is the angle it's at. But, if it actually crosses over to form a Double Cone like so: >< I would have to put a sheet of paper perpendicular from the surface of the eyepiece to find that point. This is getting much more complicated than I had envisioned, and I don't think I could get a very accurate answer if it's based on so many measurements.
Is there a simple way measure the exit pupil? Is the best way to actually measure it from the surface of the lens, or do I need a special tool?
I'm getting very discouraged, as I can't imagine that measuring from the lens would be very accurate, on the other hand none of the methods I know of are. I could simply set the lenses from both binoculars next to eachother and see which is bigger, although I had hoped to confirm more than that. I may just have to accept the published specifications, as contrary to everything I know as it is. They give the numbers 4.6 and 4.55, the 4.55 works out to about 22x. But the 4.6 works out to a different number, does this mean I can trust the listed exit pupil?
So, my quesitons are:
Is there is a simple and accurate way for me to measure the exit pupil?
Does anyone know if the listed specifications are correct?
Thanks.
Edited by Daniel_Schwartz (02/10/04 05:01 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Try measuring magnification instead. it's very easy for smaller binoculars. For 22x100s, the scale would be so far away, you would not be able to see the increments with the naked eye. For small binoculars, you can tape a yard stick or foot ruler to the wall and view simultaneously with one eye to the objective and one naked eye. You can clearly line up the magnified inch to a number of inches and get magnification. From there, exit pupil is just math.
You don't need to project it. Use a $12.95 vernier caliper from Edmund scientific to measure it right at the eye lens. Just point the binocs out a window and look for the exit pupil in the eye lens and measure.
I've never found exit pupil to vary by more than two tenths of a mm. But that means that my 12x50 binoculars that actually measure 4.4mm e.p. with a vernier, really have a magnification of only 11.3x. I should double check that magnification using the ruler method. It would be easy. For a magnified 1 inch, I should be able to line up 11.3 inches next to it with the naked eye, not 12 inches as the specs would lead me to believe.
All of this assumes objective diameter is as stated and not stopped down by a few mm with some sort of objective edge baffle or edge ring. Check objective diameters.
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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Anonymous
Unregistered
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Thanks for the help EdZ. I found an 'Educational Grade Vernier Caliper' from Edmund Scientific for $8.95. This is the only one they had, is this the right one?
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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I think that's the one i own. It's stainless steel. Hopefully you get one with clearly sharp vernier lines. You will need to calibrate it by measuring a few scales and checking. Mine reads consistently 0.4mm too big for inside measure and 0.25mm too big for outside measure. So I subtract.
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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Anonymous
Unregistered
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Thanks for the help. I just ordered it.
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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The practical difficulties outlined by Ed above added to the uncertainty of true objective diameter , and the example Ed quoted of his 12x50 possibly being only in reality a 11.3x binocular , all combine to make me wonder whether or not variations of this phenomena might contribute to perceived conclusions of "comparitive resolutions" of binoculars as typified by the "NEED" tests on the BVD review site.
After all , it would only "need" ( no pun intented ! ) a misrepresentation or misunderstanding ,brought about by design or accident , to render the apparant result misleading at best.
That is to say that if comparing Bino A with Bino B and both were allegedly 8 x 40s for argument's sake , then if Bino A was ACTUALLY a 8.3 x 40 whilst Bino B was actually a 7.8 x 38 , then it would automatically become something of an unfair competition.
Just a thought.
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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