Guide to Binocular Exit Pupil

If you divide the objective lens diameter by the magnification, you will get a number approximately between 4 and 8. This number is called the exit pupil, and represents the diameter of the beam of light that leaves the eyepiece when you hold a binocular with the objective pointed towards a light source. For example, a 7x50 binocular has an exit pupil of 50 divided by 7, just over 7 millimeters in diameter. Exit pupil also describes the image that's projected to a particular point in space.

Your eyes must be positioned at that exact spot in order for you to see the full field-of-view with maximum resolution. When holding the binoculars at arm's length, it's the small, circular image that's visible in each eyepiece lens. When your eyes are in the proper viewing position, the round shafts of light combine to create a fully detailed sight picture.

There are some other factors to consider when discussing exit pupil. As we grow older, the maximum dilation of our eye pupil diameter slowly decreases in size from about 7 or 8mm at age 20 to about 5mm at age 50. Of course, there will be some individual variation in these figures due to hereditary or environmental factors, but it's worth noting this physiological phenomenon as it relates to exit pupil size and in choosing the binocular that best meets your needs.

Another consideration is whether or not there will be any observing done under conditions that would not allow you to hold the binoculars steady, such as on the deck of a boat. Here is a situation where a large exit pupil is very useful, even for the daylight-adapted eye, because it is much easier to keep the smaller eye pupil centered in the larger exit pupil when the binoculars are difficult to hold steady.