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Quite plainly they are linked. In the case of our BTs we have the benefit of 100mm objectives. We shall shortly own EPs that provide exit pupils of 5.8mm, 4mm, 2.8mm, 2mm and 1.4mm, yielding respectively magnifications of 17x, 25x, 35x, 46x, 66x (and in most cases 65+ degrees AFOV).

I can most readily identify with daytime use when comparing these. The 66x EPs are only comfortable to use in very bright sunlight, as the small exit pupil restricts illumination of the image. As light steadily declines, such as on a summer evening, successively larger exit pupils enable illumination to be maintained. .....

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Graham,

Thanks for sharing that information. I too, enjoy using high power/large apeture binoculars for long range terrestrial viewing. Therefore I am quite interested in how far we can push magnification of various BT 100 binoculars.

My experience is with a F5-F7.5 100 achromat show 40X is near the limit for optimum daytime terrestrial viewing with non ED glass. Beyond that false color starts to intrude. The Signal to Noise(purple fringing)ratio is insufficient for me to enjoy the view. I used my friend's 20/40X 100mm 45 deg for about two weeks recently. The 40X EPs perform quite well for astronomy since I am viewing DSOs in various shades of grey. However, CA borders annoying at 40X in the daytime.

I also have a very high quality Japanese 100mm F7.5 achromat. Beatiful views of the Saturn, Jupiter and the moon at 50X+ but annoying false color in the day time.

I have similar experience to yours with exit pupil size. I dislike using binos with less than 3.125 mm exit pupil in the day time. However, at night, when my pupils are wide open I am quite comofortable using binocular or scope with sub 2mm exit pupil.

Aperture, Magnification and Power are all factors to consider in binocular selection. So is the intended use and observing style.

Erik D

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