A 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 8×42 binocular) = 5.25mm.
Under bright light your pupil may constrict down to 2mm, causing some of the light that emanates from a 5.25mm exit pupil to go unused because it’s blocked by the iris (above left in diagram). However, under dim light (like at dawn or dusk) your pupil will dilate up to 5mm or more (above right). Thus, an 8×42 binocular will deliver the maximum amount of light that your pupillary opening will accept. Next, contrast this with a 10×21 compact binocular rendering an exit pupil of 2mm:
Under bright light the compact will probably seem about as bright as an 8×42 binocular because the pupil is constricted down to 2mm (above left in diagram). However, because the pupil will dilate under dim light, a compact’s more narrow exit pupil will substantially diminish the brightness of the view (above right). This is one of the primary reasons most birders choose an 8×42; it’s the best combination of magnification and aperture, keeps the binocular’s physical size from being unwieldy, and still delivers a bright image at moderate power.
Mike McDowell
Eagle Optics Staff
Tags: Binoculars, Birding
nice post. thanks.