Exit Pupil

Genesis XD33 8x33 Binoculars

If you hold your binoculars at arms length and look at the eyepiece you will see a bright circle of light on the eyepiece. The diameter of this circle is the Exit Pupil. It effects how well you can see in different conditions and how easy the optics are to keep centered over your eye pupil. It can be calculated by taking the objective diameter and dividing it by the power. Full descriptive text along with how this will effect your view can be found in the Basics II article in the All About Optics section.