Binoculars Explained : Exit Pupil

The exit pupil is the magnified image in the eyepiece as it leaves the binocular to enter your eye and its diameter, measured in millimetres, is determined by dividing the aperture by the magnification. Exit pupil diameters will generally vary from 2mm to 7mm and, as an example, an 8 x 40 binocular will have an exit pupil of 5mm.

 When the exit pupil is smaller than your eye pupil, then the amount of light falling on the retina will be less than that collected with your normal vision at that particular time, and so the object observed will appear dim. Resolution and contrast are affected adversely, resulting in loss of clarity of the observed image.
The larger the exit pupil, the easier it is to keep the smaller pupil of your eye centred in the exit pupil. Large exit pupils permit far better viewing under conditions where it is difficult to hold a binocular steady, such as while out on the water. On a bright day, an observed image through a Classic Compact 10 x 25 binocular (exit pupil 5mm) will appear just as bright as Classic 7 x 50 (exit pupil 7.1mm) since both binoculars will take in more light than your eyes actually need. The daylight-adapted pupil (2 to 3mm diameter) is the limiting factor in observed brightness under these conditions.
You can actually see the exit pupil as a circular beam of light in the eyepiece when you hold the binocular at arm's length. The main importance of the exit pupil is how it relates to the pupil size of your eye. Your eye pupil is controlled by the iris, which acts like a variable aperture for the retina and will allow your pupil to change in size from about 2mm up to 8mm, usually depending on the brightness of the available light. When the exit pupil of the binocular is larger than the eye pupil, some of the light coming from the binocular will fall on the iris and is undetected by the observer. At twilight or in other situations of low available light, the eye pupil size will vary between 4 and 5mm. For the best performance under these conditions, a minimum exit pupil of 4mm is essential, such as that provided by a 10 x 40. At night, when the eye pupil size is at its maximum dilation (6 to 8mm), only a binocular like our Long Range 8 x 56 or 9 x 63 will meet the requirements for the optimum in observed image brightness. So you can see that it's important to consider the exit pupil of a binocular in the context of eye pupil size and the viewing time or conditions of the observer.