Binoculars or field glasses are two telescopes placed side by side and aligned to point in the same direction, enabling the spectator to use both eyes (binocular vision) when observing distant objects. Many are of the scale to be carried with both hands, although the proportions range considerably from opera glasses to large pedestal-mounted military versions.
Binoculars are typically designed for special uses. These various designs require some optical specifications which may be shown on the binocular cover plate of the prism.
As the first number in the binocular definition, the magnification is the measure of the focal length of the target separated by the focal length of the eyepiece. This brings the magnifying power to the binoculars. For eg, a magnification factor of 7, creates an image 7 times greater than the original seen from that point. The desired level of magnification depends on the intended use and is a permanent, non-adjustable feature of the system in most binoculars. Hand-held binoculars usually have magnifications ranging from 7x to 10x, making them less sensitive to the effects of trembling hands. A greater magnification leads to a narrower field of view which can require a tripod to preserve image stabilization. Any adapted binoculars for astronomy or military use have magnifications from 15x to 25x.