Figure 5-16. Function of collective lens.

TM 9-258

b. Secondary Functions. This lens also shortens the

eye reflief (distance from eyelens to eye) of the

instrument by shifting the plane of the exit pupil (in which

the eye must be located to view the full field) closer to

the instrument. It is not used, therefore, in rifle scopes or

gunsights where long eye relief is essential to' avoid

injury from the recoil.

5-13.

Functions of Eyepiece or Ocular.

a. General. An eyepiece in its simplest form

may consist of a single lens (simple magnifier) which

presents to the eye an enlarged virtual image of the real

image formed either by the objective or the erectors as

illustrated in figure 5-17. The usual apparent field

Figure 5-17. Function of eyepiece.

provided by an eyepiece is 400 to 500. A wide angle

eyepiece is one fully corrected to the edges and of

b. Function of Elements. In a simple eyepiece

sufficient aperture so that in a system of given eye relief

using two lenses, an eyelens next to the eye provides all

a wide field of up to 75. may be provided. Because

or most of the magnification depending on the location of

magnification is angular, this apparent field limits the true

the filed lens relative to the real image. The field lens (fig

field of the instrument for any certain power. For

5-18), if in the image plane, has no effect on

example, a 4-power instrument with an eyepiece

magnification but acts as a collective lens providing

providing a 400 apparent field will have a true field of

balanced illumination of the field. Without the field lens

100. This will be discussed fully in paragraphs 5-21

principal rays from the edge of the field would miss the

through 5-31.

eyelens and leave the edge of the field dark. With the

field lens in the image plane, however, any dust on its

NOTE

surface lying in the image plane will be in sharp focus.

The power of an eyepiece is inversely

For this reason, unless there is an etched reticle on this

related to its focal length; i.e., high-

surface, the field lens usually is displaced slightly from

power eyepiece is one of short focal

the image plane thus contributing to the magnification.

length. It is thus possible to replace an

Another reason for not placing the field lens in the focal

eyepiece with one of shorter focal length

plane is that this is the position normally occupied by the

and increase the power of the

reticle in those instruments emplying reticles with the

instrument.

reticle pattern etched on glass or a

5-9