Re: Binoculars w/reduced effective apertures

ezarenski_at_gilbaneco.com
Date: 01/21/05 

Date: 21 Jan 2005 05:49:01 -0800

Sketcher wrote:
> On 20 Jan 2005 14:04:32 -0800, ezarenski@gilbaneco.com wrote:
>
> Focus the binoculars for infinity. Suspend a 56mm bar (the bar could
> be cut from posterboard, etc.) right in front of and as close as
> possible to the objective. Then examine the exit pupil. Adjust the
> bar so that it bisects the exit pupil. Make additional adjustments
> until you can either see light or a gap (while examining the exit
> pupil) at *both* ends of the bar or no light or gap at either end of
> the bar. In the former case your effective aperture is greater than
> 56mm. In the latter case your effective aperture is no more than
> 56mm. Repeat the experiment with bars of larger or smaller lengths
> until you find a length that just barely prevents light from entering
> at either end.

If you happen to cut the right size bar this would give you the
effective apperture. With several trials, you should get it.

The procedure we have outlined to test for the several aspects involved
in this are:

You cannot simply measure exit pupil, since that does not confirm
magnification. So first magnification must be verified.

Test magnification. Observe a scale mounted on the wall across the room
with one eye to the binocular and one eye unaided. Use white cards,
business cards and tape work fine, to tape off a one inch unit.
Compare this magnified one inch to what is seen unaided. Use a movable
card to mark off the extent of what is seen unaided to compare to the
magnified unit. This would give you close focus magnification. An
attempt should be made to compare that magnification to a test at
distance, since magnification varies with distance of focus. About 3
out of 10 binoculars that I've tested vary by as much as 3% to 7%.
I've never found a binocular that had a lower magnification than
stated. But binoculars with higher magnification create smaller exit
pupils.

Measure exit pupil with binocular at close focus and then focused at
infinity. For this you need a vernier caliper, or at the very least a
precise scale that has divisions finer than mm and a magnifying glass.
You need to record exit pupil as accurately as you can, preferably to
0.1mm. A quick observation of the exit pupil is the first clear sign
that something might be amiss. If you have a 9x63 binocular and you
can quickly eye it up and only see a 6mm exit pupil, you know you have
a problem. But if a quick look seems to show you a 7mm exit pupil, you
may not have any problem at all.

Do the math with exit pupil and magnification. You can determine
effective aperture from these. When you are done, you will have
recorded measurements for three important aspects of you binocular.
edz

Relevant Pages

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  • Re: How many pixels?
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  • How many pixels?
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