Binoculars are a popular means of both terrestrial and astronomical observation. Binoculars are essentially two telescopes mounted side by side. They have the advantage of providing an impression of depth of field or a 3-D view for terrestrial or lunar observing.
Binoculars are specified with two numbers, such as 8 x 42. The first of these numbers represents the magnification. The second represents the objective lens diameter measure in millimeters. Larger objective lenses are advantageous for observing in low lighting. Binoculars with objective lenses of 50 mm or larger are often intended for use as astronomical binoculars. Popular sizes include 7 x 50, 8 x 56, and 9 x 63. Note that the ratio of those three sets of numbers are all approximately equal to 7. The exit pupil of a telescope or binocular is equal to the entrance pupil (objective lens size) divided by the magnification. An exit pupil of 7 mm is considered ideal for astronomical binoculars because this is the maximum dilation of the human eye pupil; an exit pupil of 7 mm produces the brightest possible image.
When it comes to the magnification of binoculars, there can be too much of a good thing. Most of us can easily hand hold a binocular for a steady image with 7 or 8x magnification, but that task becomes harder as the magnification increases above 10x. We recommend that any binocular with 15x or more magnification be used only in conjuction with a binocular mount.