Objective LensesThe objective lenses of binoculars are the front lenses. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular binocular. Hence, a 7×42 binocular has an objective lens of 42mm. The diameter of the lens determines the light gathering ability of the instrument, with the greater light gathering ability of a larger lens translating into greater detail and image clarity. This is especially useful in low light conditions and at night.
The main (objective) lenses of a binocular serve to collect light, thereby enabling the high-resolution observation of distant objects. In a quality binocular each objective lens typically is manufactured of two separate glass elements, the so-called crown and flint elements. The refractive specifications of these elements permit the objective lens to image objects free of false colors.
The larger the diameter of the front objectives, the more light can enter the binoculars and be focused to your eyes. Thus, other things being equal, you can see better in dim light with binoculars that have large front lenses. For example, 7×50 binoculars are often called “night glasses” because they seem so bright in dim light. Read more about this under “Exit pupil.” Of course, a large front objective makes for larger, and heavier, binoculars. This is why the compact binoculars generally have relatively small front lenses.
