Lens. A transparent object, usually a piece of optical glass, having two polished surfaces of which at least
one surface is curved. It is shaped so that rays of light, when passing through it, are made to converge or diverge. A lens
can be made from a single piece of optical glass or it can be made of two or more pieces of glass cemented together.
Some examples of lenses used in optical systems are listed below:
Erector lenses placed between the focal planes of the objective and the front focal plane of the eyepiece.
Their function is to produce an erect or upright image which would otherwise be inverted and reverted.
The eyelens is located nearest the eye and is used to direct the light rays into the eye. This lens does
the actual magnifying of the image and can affect the quality of the image as seen by the eye.
The field lens, located in the eyepiece nearest the objective, is used to gather light from the objective or
erectors and converge it into the eyelens.
The objective lens is located in an optical system farthest from the eye. It receives light from the object
being viewed to form an inverted and reverted image.
A window is a piece of glass with no refracting power (with parallel surfaces). Its function is to admit light
into an optical instrument and to keep out dirt and moisture.
Diaphragm or Stop (fig. 9-7). A diaphragm is a flanged or plain ring, with a limited aperture, placed in an
optical system at any of several points. They cut off marginal rays of light not essential to the field of view. The rays of
light which cause aberrations, glare, and ghost images are eliminated by the use of diaphragms. There are three general
uses of diaphragms which are defined as follows:
Aperture stops are diaphragms used to limit the aperture of light gathering power of the instrument.
Field stops are diaphragms placed in an image plane to reduce aberrations in the instrument.
Antiglare stops are used to eliminate reflections from the sides of the instrument tube and consequent
glare in the fields-of-view.
Figure 9-7. Diaphragm (Stop) Location in an Optical System
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