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What would cause the exit pupil brightness to be nonuniform

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May be this is why i see a brightness difference between my 10x56 Victory FL and my 10x50 Leica in gloomy daylight

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from the article:
Why is it we may not be able to see the difference in brightness?
Consider this. If you have been using binoculars with an exit pupil larger than your eye pupil, all along, your eye pupil has been creating an effective aperture. You cannot get a greater amount of light (a wider beam of light) into your eye than can be allowed thru your pupils. If you’ve been using a 10x70 binocular with a 7mm exit pupil and your eyes dilate to only 5mm, you have never received a larger beam than 5mm into your eyes. Therefore, all along, the binocular has been performing as if it were a 10x50.

So you might ask then, in bright daylight viewing, if eye pupils dilate to only about 2.5mm, would it not be the same to use a 10x25 as opposed to a 10x50?

What differences can be seen in terrestrial viewing?
Let's assume a terrestrial viewer is using a 50mm binocular with a 5mm exit pupil (10x50). Now assume the observer’s eyes in daylight can dilate to only a maximum of 2.5mm. The difference between the full aperture exit pupil of 5mm and the effective aperture exit pupil as a result of the 2.5mm eye pupil, for a 50mm binocular, observing an object at a distance of 100 feet is 5.5 arcseconds of angular resolution, a linear dimension on the object of interest of only 0.03 inches, approximately the width of a thin vein on a leaf. While this is a very real number, it is not likely something most people are able to notice.

What happens in low light terrestrial viewing where many seem to prefer a larger exit pupil because it seems brighter?

Now we need to go back to the cardinal rule that states exit pupil controls effective aperture. There would be no more light delivered by either binocular if eye pupil is 2.5mm, BUT, as seen through a 2.5mm eye pupil, there may seem to be a difference in the two binoculars, and you might be able to see this difference. The 10x25 binocular is using the entire objective lens diameter and we have seen from vignette studies, many (if not all) binocular systems block (vignette) portions of the light from the periphery of the objective lens. The 10x25 is using the entire diameter of the lens to light the exit pupil and the light from the outer portions of the objective is reduced in the exit pupil due to vignette. Not all of the light gathered by the 10x25 reaches the eye.

Most vignette studies show that approximately the central 50% of an objective lens provides 100% illumination of the exit pupil. The light delivered from the area outside the central 50% of the objective does not all reach the exit pupil. In the 10x50 binocular, stopped down by the daylight contracted eye pupil to 10x25, only the central 50% of the objective diameter is putting light into the eye pupil. The key is this 2.5mm exit pupil is 100% illuminated from the central portion of a stopped down 50mm lens. A stopped down 10x50 is providing a lot more illumination to the same size 2.5mm exit pupil than a full aperture 10x25. That would account for a significant difference in apparent brightness between the two, even though both situations have a 2.5mm exit pupil. As a bonus, that same 10x50 binocular will serve the user better under lower light conditions when eye pupil is enlarged to possible 3mm or 4mm.



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Edited by EdZ (06/16/07 10:24 AM)

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