Exit pupil

The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission).

In daylight, when the eye's pupil is only 4 mm in diameter, over half the light will be blocked by the iris and will not reach the retina.

The maximum pupil size of a human eye is typically 5–9 mm for individuals below 25 years old and decreases slowly with age as shown as an approximate guide in the table here.

[1] The exit pupil can be visualized by focusing the instrument on a bright, nondescript field, and holding a white card up to the eyepiece.

A clear vial of milky fluid can be used to scatter light rays exiting the eyepiece, making their paths visible.

Digital image sensors often have a limited range of angles over which they will efficiently accept light, especially those that use microlenses to increase their sensitivity.

[6] The closer the exit pupil to the focal plane, the higher the angles of incidence at the extreme edges of the field.

For this reason, many small digital cameras (such as those found in cell phones) are image-space telecentric, so that the chief rays strike the image sensor at normal incidence.

Single lens imaging with the aperture stop. The exit pupil is the image of the aperture stop formed by the optics behind it, and the location and size of the pupil are determined by chief rays and marginal rays .
The image side of the lens of an SLR camera; the exit pupil is the light area in the middle of the lens.
The aperture of this optical system is the edge of the objective lens. The exit pupil is an image of the aperture made by the optics following it; divergent rays from each point in the aperture plane come together again in the exit pupil.
The small exit pupil of a 25×30 telescope and large exit pupils of 9×63 binoculars suitable for use in low light
The exit pupil appears as a white disc on the eyepiece lens of these 8×30 binoculars. Its diameter is 30 ÷ 8 = 3.75 mm.