For telescopes, the optical diameter (also known as aperture) is the size of a telescope's main light-collecting lens or primary mirror, measured in millimeters or inches. Telescopes with larger optical diameters collect more light, which leads to an increase in brightness and image resolution compared to smaller instruments.

For binoculars, the optical diameter (also known as objective lens diameter) is the size of each of the front-facing objective lenses of a binocular measured in millimeters. Binoculars with larger objective lenses collect more light, which increases image resolution and brightness. Binoculars with larger objective lenses are recommended for low light situations, and binoculars with at least 50mm or larger objective lenses are recommended for pleasing astronomical observations at night.

The diameter of the circle of light exiting the binocular. Exit pupil is calculated by dividing the objective lens diameter by the binocular magnification. For example, a 10x50 binoculars has an exit pupil of 5mm (50/10=5). Knowing the exit pupil of a binocular can help determine the optimal binocular fro your needs. If a binocular exit pupil is larger than the dilation of the user's pupil, resolution is reduced since the maximum amount of light is not entering the eye. Typically under true dark adaptation a healthy human eye will dilate to approximately 7mm. This number can decrease with age and health factors.