The exit pupil can be seen by holding the binoculars at arm’s length and looking through the eyepieces. The cone or pencil of light you see is the exit pupil. The diameter of the exit pupil determines how much light is transmitted to your eye. The actual diameter of the exit pupil is computed by dividing the diameter of the front objective lens (in millimeters) by the magnification of the binocular. So, in the case of a 7×50 binoculars, the diameter of the exit pupil is 7.1 and in the case of a 10×25 it’s 2.5.
For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated pupil of the human eye (in other words, in the dark). In the case of young people, this is about 7 mm, but less in the case of elderly people. If the cone of light streaming out of the binoculars is larger than the pupil it is going into, any light larger than the pupil is wasted in terms of providing information to the eye. This means that a lot of information is wasted in bright daylight, when the human pupil is dilated to about 3 mm (that is, if the exit pupil of your binoculars is more than 3 mm).
However, even that being true, this is a compromise one has to settle for: A bigger exit pupil makes the instrument so much easier to use: No struggle to get your eyes inside the cone of light coming through the binoculars, whereas this is not that easy with a binocular with a small exit pupil. Finally, a bigger exit pupil comes into its own when the instrument is used in dim conditions, when the human pupils are fully dilated. This is when the maximum amount of light coming through the exit pupil is desired and that is provided with a big exit pupil, not a small one. I like to use my 10×25 during the day, but when it starts getting darker, I switch to my 10×50, in a situation where I have both available. The difference is what I see, is remarkable.
The ability of a binocular to transmit light in terms of the exit pupil is expressed as its performance on the relative brightness index (RBI). It is computed by squaring the exit pupil. For example, 7×35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5×5=25). A RBI of 25 or greater is considered good for use in dim light.