Field Glass and Prisms What Do They Have in Common?
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Field Glass and Prisms What Do They Have in Common?

This explains how field glass are very similar to prims.

Field glass is a small double telescope with a separate viewing tube and set of lenses for each eye. The simplest field glass has the two tubes joined at the same time side by side. It can be adjusted by a knob which moves the lenses in each tube closer together or spread apart. This simplest field glass has been replaced by the prism binoculars. In such binoculars, the light in each tube as reflected by two prisms before it reaches the eye. The light enters the binoculars through the magnifying lens at the front and strikes one prism and is reflected frontward. Then the light strikes another prism and is reflected back to the lens or lenses in the eye piece.


The prisms inside such a binocular are useful in three ways. They turn right side up the reversed image which is brought in by the magnifying lens. They help to make the binoculars smaller because the light can travel more in being reflected from one prism to another than it could in a direct line. In most binoculars the magnifying lenses can be set father apart than the eye pieces are. This makes for a better stereoscopic vision at distances. Good binoculars have achromatic lenses which compensate for the bending of the different light rays. Some have specially coated lenses to reduce glare.

Field glasses are made in different magnifying powers. A field glass which magnifies the diameter of an object to six times its actual size increases the surface of the object six times six or thirty six times. A power of nine magnifies an object eight one times. Naturalists and military officers often use a field glass with a power of twelve which increases the size of an object to 144 times.


The earliest binoculars are called Galilean binoculars. They were developed roughly around early 1800’s. The binoculars can be as small as the opera binoculars to big as the ones tourist use to see out Empire state building. The binoculars of the 1800’s used the roof prism which means the object lenses are just about in line with the eye pieces. There is also a Prism called Porro-prism which produces brighter images than roof prisms with same magnification size.


Porro prism uses the Z-shape configuration to show the image. These binoculars are wide the objective lenses are well separated but offset from the eye pieces. Therefore this gives added benefit of folding the optical path so that physical length of the binoculars is less than focal length of the object and wide spacing of the object which gives better depth perception while viewing the object.


All text, descriptions, and pictures used here by Sunshinelady are owned and copy written by Sunshinelady and may not be used without the express written permission of Sunshinelady!

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