What is exit pupil, and why is it important?
Though the objective lens diameter determines how much light will be gathered by a given binocular, it only partially determines how bright an image you will see. Another factor besides aperture is in play to produce a bright image: its called exit pupil. In general, a larger exit pupil equals a brighter image, providing you do not exceed your maximum dilated pupil size. An example is that if you choose a binocular with a 7mm exit pupil but your eyes only dilate to 5mm, you are effectively wasting a good percentage of the aperture of your binocular. In order to achieve maximum binocular performance, exit pupil size should be carefully considered and matched to your pupil size and observing conditions. Exit pupil is one of the most neglected and also the most important binocular specifications, and in most cases it is easily determined by dividing the aperture by the magnification. In other words, a 10×50 binocular would have a 50/10=5mm exit pupil. As it turns out, a roughly 5mm exit p
