Exit pupil is a circle from which the light is being transferred to your eye through the optical product. When you holdthe optics a bit far away from your eyes towards a light, exit pupil can be seen as a bright circle in the center of each eyepiece. The larger the exit pupil the more light can reach the eye and the image appears brighter. This is why exit pupil plays an important part in the optical products in poor light conditions at dawn or dusk. Size of the exit pupil also determines how comfortable viewing through an optical product really is. An important factor is also the size of the eye box, which is a space where the eye still has an entire picture, without any tunnel vision or blurry edges. Bigger eye box means more flexibility of the eye position and therefore more comfortable viewing, because the eye can move in several directions within the eye box and still obtain a full image.
The diameter of the exit pupil is calculated by dividing lens diameter with magnification. E. g. 8x50 binoculars have an exit pupil in the diameter of 6.25 mm.
To ensure a brighter image, the eye pupil in low light conditions has to be at least as big as the exit pupil. This way there’s no loss of light and the image is as bright as possible. However, the maximum diameter of the eye pupil depends on age. At night, children’s eye pupils can widen up to 7 mm, while with aging they decrease to a maximum of 4 mm. So if the viewer’s pupils can only be open up to 4 mm, the 7 mm exit pupil cannot be fully utilized. It may contribute to more comfortable viewing, but not to brighter image.
In daylight, when the eye pupil is open up to 3 mm, all the optics with exit pupil bigger than 3 mm are equally bright. For example, the 8x30 binoculars with 3.75 mm exit pupil are no brighter than 8x56 binoculars with 7 mm exit pupil. Those with 7 mm are however more comfortable to use, since they’re less sensitive to the eye position (they have bigger eye-box).