Binoculars and Binocular Mounts
Binoculars are available in a wide range of magnifications and sizes. Binoculars are almost always described using two numbers, such as 7 X 50 or 8 x 56. The first of these numbers refers to the magnification. The second number represents the diameter of the objective lenses measured in millimeters. Larger binoculars gather more light and thus are more effective in low light situations. You will notice below that binoculars used for astronomy commonly have very large objective lenses. General purpose binoculars have smaller objective lenses, with the range of 30-42 millimeters being the most common.
Binoculars fall into one of two general design categories: porro prism and roof prism. Porro prism binoculars feature the more familiar dog-leg bend in the middle of each ocular. They tend to be larger and heavier that the newer roof prism design, but are favored by astronomers. Roof prism binoculars present more of a straight through appearance. They are favored by birders, naturalists and hunters for their compact design.
Selecting the right binocular for hand-held use often involves a compromise between image brightness and the size and weight of the glasses. Birders, hunters and naturalists value lightweight binoculars that can be easily hand held. Amateur astronomers want all the light gathering power we can get and are often willing to accept bulky binoculars to do the job. Often these larger binoculars will require a mount in order to maintain a steady image. A magnification of 10X is about the maximum that most people can tolerate in a hand held mode. Beyond that magnification the image may become very shaky indeed.
Mid-sized binoculars having objectives of 30-40mm represent the mainstream of the binocular market. Their size and weight are easily handled by almost anyone, and most women prefer them to larger sized binoculars. Their objective size gathers enough light to work well in most lighting situations.
Full sized binoculars provide increased image resolution and higher performance in low level lighting situations. These attributes make them popular with both naturalists and hunters alike. Their size and weight can be difficult for some people to hold steadily.
Compact binoculars are designed to fit in a jacket pocket. Their relatively small objective size limits their usefulness to well lit situations.
Binocular viewing can be an enjoyable introduction to astronomy, but there is an upper limit to the magnification or the weight of the binoculars that can be effectively hand held. Binocular mounts overcome these limitations.