exit pupil

The exit pupil is the tiny circle of light you see when looking through binoculars or a telescope, representing the light beam that exits the device and enters your eye. It’s calculated by dividing the instrument’s aperture (lens diameter) by its magnification. For example, a telescope with a 100mm lens at 25x magnification has an exit pupil of 4mm. A larger exit pupil allows more light to reach your eye, resulting in a brighter, clearer image in low-light conditions. It also influences how comfortable viewing is, especially for extended periods.