An exit pupil binocular is another factor apart from the magnification that most people associate with a binocular while buying the binoculars.
How to see the exit pupils of the Binoculars?
If you hold the pair of binoculars at an arm’s length then you will see a small circle of light in each the eyepiece. This circle which you see is the exit pupil. As a thumb rule the exit pupil diameter is the obtained by dividing the aperture in millimeters by the magnification.
For example, if the specification is 7×35 for the binoculars then the exit pupil of 5mm.
What does it mean to me if the exit pupil of a binocular is less or more?
The exit pupil of the binocular should match the entrance pupil of the human eye. In normal cases the human eyes’ entrance pupil for a young adult is 5-7mm.
This is important to know because generally that determines the image’s brightness.
If your entrance pupil diameter is less than the exit pupil diameter then outer edge of the small circle of light that is coming out of the exit pupil will fall on the sides of your entrance pupil and you will see a dimmer image.
In case your entrance pupil diameter is larger than the exit pupil diameter of the eyepiece then you will be able to get all the light and will see a brighter image.
That will mean that you should make sure that the entrance pupil of your eye is equal or more than the exit pupil diameter of the eyepiece.
Young adults have about 7mm diameter in low light conditions. As you age the entrance pupil diameter will reduce.