Binoculars Explained : Objective Lens

The second number of a binocular designation refers to the diameter, in millimetres, of the front, or objective lens. The diameters usually range from 20 to 50 millimetres and this number will almost always be directly related to the size of the binocular. So called "giant binoculars", used mainly for astronomical purposes, may have up to 70 or 80mm objectives, while compact models will usually be 20 to 25mm in diameter. The objective lens size, or aperture, determines the amount of light that will enter the optical system.

 Once the objective gathers the light into the binocular, other factors determine how much light is transmitted through the optical system and all of these factors, including the aperture, combine to determine the brightness and clarity of the image you actually see. These other factors include magnification, exit pupil size, available light, the presence and type of anti-reflection coatings used, and the size and quality of the optical glass and prisms used in the construction of the binocular.
A larger objective lens will gather more light and theoretically provide greater detail and clarity of the image. This is especially true under low light conditions. Since the amount of light that will enter the objective lens will vary by the square of the change in the radius, a small difference in objective lens size will have a greater impact on the light gathering ability than one might first suspect.

(The common assumption that the size of the objective lens will determine the field of view is seldom true as field of view is controlled largely by the optical design of the binocular.)