Telescope Reviews: Brightness in Exit Pupil

This excerpt is from my most recent binocular review titled Binocular Tests and Comparisons. It should help answer Kenny's question why some exit pupils appear brighter than others. It may not be the only answer, since there will be spin-off discussion about field of view. But this answers the basic question, even if Apparent field of view is constant.

Brightness of the image
A binocular with a larger exit pupil will have a brighter image. Well, most of the time. Often we read of a user comparing the apparent brightness of the image in the exit pupil from one model binocular to another. Assuming equal size aperture, sometimes it seems difficult to understand how binoculars with equal size exit pupil may differ in brightness or why it might be that a smaller exit pupil binocular may appear to have a brighter image. Standard calculation tells us the larger exit pupil provides a brighter image. But that is the simple calculation, and all is not always what it may simply appear to be.

Assume for example two different 100mm binoculars one with a 4.5mm exit pupil and the other with a 4mm exit pupil. Everything tells us the binocular with the 4.5mm exit pupil should give a brighter image. Why then might it be possible for the 4mm exit pupil to give a brighter image than the binocular with a 4.5mm exit pupil.

In addition to the potential light lost to the exit pupil based on the quality of the coatings and the number of uncoated or single coated surfaces, the following may also contribute to light lost from the exit pupil. Studies have shown that not all binoculars deliver all the light expected based on the simple calculations of aperture area and exit pupil. The brightness of the exit pupil is considerably affected by vignette in the binocular system, especially in the prisms. Even for some of the best binoculars, vignette is present. The amount present can cause considerable differences between models of the same sizes and between varying sizes of objectives producing the same size exit pupil. Total brightness cannot simply be assumed based on expected transmission figures or light gathering area and exit pupil.

If all the light that enters the front prism hole does not exit the back prism hole, it is bouncing around inside the prisms. This does several things. It causes what is known as vignette and it reduces contrast in the image. It will also reduce the brightness of the image in the exit pupil. So it is possible to have two equal sized binoculars with equal size exit pupil and yet one binocular can have a brighter image than the other. Although present even in closely sized and quality binoculars, it may not be as apparent as it sometimes is between high quality and cheap binoculars. But it is a factor.

end

I think we perceive brightness in concert with contrast. Two binocular that have the same lens area, same magnification and therefore same size exit pupil, even so far as to say the same quality coatings, may not appear equally as bright if one looses more contrast than the other. There are many ways that can happen internally and any loss of contrast will cause one image to appear somewhat less desirable than the other.

edz

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