Binoculars Explained : Aperture
| Magnification | Aperture | Field of View | Depth of Field | |
| Size & Weight | Eye Relief | Transmittance | Exit Pupil | |
| Optical Design | Focusing | Twilight Factor | Phase Correction | |
| ED (Extra Low Dispersion Glass) |
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.
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.
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.
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.
(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.)
(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.)