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Old Wednesday 15th April 2015, 15:14   #23
henry link
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Join Date: Feb 2004
Location: north carolina
Posts: 4,238
Originally Posted by OPTIC_NUT View Post
I am a bit baffled by the exit pupil theory,
because bad examples of that kind of glare have it smeared across the field,
not just near the edges. It cannot be wiped away by smaller irises.
What falls on the surface of the eye is the exit pupil, which is not a smear but a nice sharp image of the objective lens and the interior of the binocular around it. The "smearing" occurs when the exit pupil is defocused by the lens of the eye as it is projected onto the retina.

The photos below show an example of bad veiling glare in a binocular (Swarovski 8x30 Habicht). The left photo shows the exit pupil with a glancing reflection coming from the edge of the objective lens cell. The right photo shows what that reflection looks like once the eye is placed at the eye relief distance in normal viewing (or in this case when the camera lens is placed at the eye relief distance).

These photos were made on an overcast day with the binocular pointing into a dark area. The veiling glare comes from sky light falling on the lens cell at just the right angle to reflect back to the eyepiece. Under those conditions the eye could easily be open to more than 4mm, so the edge of the 3.75mm exit pupil along with the thin crescent of metallic reflection would enter the eye and appear as a "smear" covering the bottom half of the field. The very same reflection at the edge of a 7mm exit pupil would fall harmlessly on the iris of the eye.

I think it's worth noting that this particular binocular appears from the front to be exceptionally well baffled. There is a long ribbed and well blackened cone between the objective lens and the prism shelf which completely blocks any view of the binocular interior and the prisms from the front. Only the first aperture of the prism shelf is visible through the objective lens, but all that impressive looking baffling is ineffective against veiling glare because the front of the cone is a little too large, allowing the objective lens cell to be exposed when viewed from the eyepiece end of the binocular.

I highly recommend the use of a magnifier to anyone who wants to understand veiling glare in binoculars. Use the magnifier to examine the interior from the eyepiece end under the conditions that cause glare. You'll see exactly where it's coming from.
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Last edited by henry link : Wednesday 15th April 2015 at 15:28.
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