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 is measured in millimeters. An 8×21 mm 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 binocular’s 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 binocular’s eyepieces (or oculars). The larger the exit pupil, the brighter the binocular image becomes. Keep in mind, however, that larger binocular lenses means larger binoculars.
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