Binoculars are used, like a small telescope, to view distant objects. They employ an optical system of lenses and prisms to produce an enlarged erect image. The ocular and the objective lenses provide the magnification and illumination. Between them is a pair of 45-90-45 degree prisms so arranged that the light passing through the binoculars is internally reflected four times, making the image erect..... Three factors are involved in the usefulness of binoculars--magnification, field of view, and light-gathering power. Magnification must be suited to the purpose. Any movement by either the observer or the observed is magnified at high power..... Higher magnifications require a tripod or other support. Field of view is largely determined by the ocular lenses. The diameter of the objective lens determines the light-gathering power--the larger the better if binoculars are used at night or in shady woods. Binoculars have central or individual focus (central preferred). They range in magnification from about 2 to 20. Each binocular has an identification mark such as 8 x 30 or 7 x 50. The first number is the magnification, the second the objective diameter in millimeters. An 8 x 30 glass has slightly greater magnification but distinctly less light-gathering power than a 7 x 50. [p. 76]
Microscopes, projectors, and enlargers are similar in principle, but they differ in purpose and design. In each, a positive lens forms a real image of a brightly illuminated object. With projectors, the image is caught on a screen; with microscopes, it is viewed through an eyepiece; and with photographic enlargers, the image is projected on light sensitive paper, where it is recorded in semi-permanent form.
[Rainwater, Clarence [Prof. of Physics, San Francisco State College, Original Project Editor Herbert S. Zim]. Light and Color. Golden Press, NY, Western Publishing Company, Inc., 1971.]