...your guide to using optics.

What about the light gathering ability of my binocular? 

Your eye is uniquely designed to gather more or less light as conditions change, but your binoculars are not. Since the light gathering ability of a binocular is fixed, it is important to select a model that best meets your eye's need for light as viewing conditions grow darker.

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.
5.25mm Exit Pupil

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 8x42 binocular will deliver the maximum amount of light that your pupillary opening will accept. Next, contrast this with a 10x21 compact binocular rendering an exit pupil of 2mm:

2mm Exit Pupil

Under bright light the compact will probably seem about as bright as an 8x42 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 8x42; 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.

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