Binoculars are an optical devices constructed to allow the magnification of objects without sacrifice of depth perception. As their name implies, binoculars are optical devices intended for use with two eyes in order to preserve human stereoscopic vision. In reality, binoculars are simply a pair of telescopes strapped side by side. The physics of the telescope must be understood before one can appreciate the clever engineering in binoculars.

Good binoculars require the following properties: an appropriate level of magnification; a bright image; a good depth of view; and reasonable portability. If we consider these four properties in terms of the telescope we will start to appreciate the limitations forced on us by the physics of the instrument.

First consider magnification. Binoculars use two lenses (objective and eyepiece). The magnification of an object by the use of two lenses totally depends on the focal lengths of the objective and...