AFTER using sporting optics — riflescopes, binoculars and spotting scopes — for seven decades, I have gained a reasonable knowledge of these products and how they work. It is indisputable that optical instruments vary considerably in quality according to their price; there are excellent ones, very good ones and poor ones.
Hunters and shooters reading magazine articles about optics are often confused about the system of terms which forms the nomenclature used to describe how sport optics work. Here’s an explanation to help you understand the terms generally applied to riflescopes.
EXIT PUPIL
The exit pupil is the small disc of light that can be seen in the eyepiece of the scope when the optic is held at arm’s length and pointed at the sky or a bright light. The size of the exit pupil is a function of both magnification and objective lens diameter. To determine the diameter of the exit pupil, simply divide the diameter of the objective lens in millimetres by the magnification. Obviously, the exit pupil’s size will vary