Symmetrical

TM 9-258

i. Symmetrical. This eyepiece employs two

portion of the field providing a more uniformly.

illuminated image. The third is a double convex doublet

doublet (compound) double convex lenses, with the

eyelens with a slight curvature facing the eye and the

greater curvature (crown side) of each lens facing that of

greater curvature facing the collective lens and the

the other. The symmetrical eyepiece illustrated in B,

convex surface of the field lens. The periscope using

figure 4-5, provides long eye relief (related to low power

this eyepiece is the battery commander's periscope.

and large exit pupil). It has a larger aperture than a

k. Orthoscopic. This eyepiece is illustrated in

Kellner of the same focal length providing a rather wide

B, figure 4-7. It employs a planoconvex triplet field lens

field. It is used commonly in rifle scopes and gunsights

and a single planoconvex eyelens with the curved

requiring long eye relief between the eye of the gunner

surface of the field lens facing the curved surface of the

and the eyepiece when the weapon recoils, and low

eyelens. It is free of distortion and is useful in high-

power with a large field.

j. Erfle. The Erfle eyepiece illustrated in A,

power telescopes because it gives a wide field and high

magnification with sufficient eye relief. It is useful in

figure 4-7 employs three elements. One is a concavo-

range finders to permit use of any part of the field. It is

convex doublet field lens. Another is a symmetrical

so named because of its freedom from distortion.

collective center lens which adds to the power of the

eyepiece by shortening the focal length and improves the

illumination of the outer

4-9