The exit pupil is the magnified image in the eyepiece as it leaves the binocular to enter your eye. It is an indicator of how well you will see an image through your binoculars on a bright day, at twilight or at night. Almost all binoculars gather more light than is needed by your eye for viewing in bright conditions. For the best viewing at twilight, you'll want a binocular with a minimum exit pupil of 4mm. A more diminutive but technical definition of Exit Pupil is the diameter of the shaft of light (expressed in millimeters) that exits a binocular’s eyepiece that can enter through your eye’s pupillary opening. Exit Pupil is calculated by dividing a binocular’s aperture by its magnification, e.g. 42mm / 8x (for an 8x42 binocular) = 5.25mm.