What does diameter mean?
The amount of light passing through the different lenses of the binocular depends on the diameter of those lenses. The objective lenses are located at the front of the binocular. The diameter of the objective lenses are measured in millimeters. An 8x21mm binocular has an objective lens diameter of 21 millimeters. The larger the diameter of the lens, the more light they will gather. More light means a brighter image of greater detail and clarity. The size of the exit pupil also affects the brightness of an image. The exit pupil is the diameter of the beam of light, in millimeters, that passes through the eyepieces (oculars) of the binocular. The larger the exit pupil, the brighter the image becomes. Keep in mind however that larger lenses mean larger binoculars, which can become bulky and cumbersome. Most people are willing to sacrifice the slight differences in brightness in return for a compact, more manageable model.
