Figure 4-4. Typical light paths through field lens and eyelens of eyepieces.

TM 9-258

e. Types of Oculars or Eyepieces. Types of

b. Construction. The lens of the eyepiece

nearest the eye is called the eyelens. The eyelens does

eyepieces used in optical fire-control instruments are

the actual magnifying of the image and can affect the

discussed in the following subparagraphs.

They

quality of the image as seen by the eye. The lens in the

represent general types rather than the specific

eyepiece nearest the objective is called the field lens.

eyepieces of instruments inasmuch as a specific type of

The field lens gathers the light from the objective and

eyepiece generally is modified to make it best suited for

converges it into the eyelens. If it were not for the field

a particular instrument.

f. Kellner. The Kellner eyepiece (achromatized

lens, much of the marginal light gathered by the objective

would not be brought into the field of the eyelens (A and

Ramsden, g below) consists of two lenses, field lens and

B, fig 4-4).

eyelens; the eyelens is corrected for color and is a

c. Relation of Eyepiece to Optical System. The

doublet or compound lens with the crown forward and

objective takes nearly parallel rays of light from a distant

the flat flint facing the eye. For full chromatic correction,

object and brings them to a focus turning them into

image lies in plane surface of field lens, but to use with

angular rays. The eyepiece takes these rays, which are

reticle is positioned forward of field len as illustrated in A,

diverging after having been brought to a focus, and

figure 4-5. In this case, some color correction is

directs them into the eye.

sacrificed. To reduce aberration a dense barium crown

d. Magnification. Magnification by the eyelens

and light flint are used in the eyelens. This ocular gives

an achromatic and orthoscopic field that in some models

or eyelenses may be fixed of variable, or there may be

is as large as 500. Its most serious disadvantage is the

no magnification. Some instruments are provided with

pronounced ghost resulting from light reflected

changeable eyepieces to secure different degrees of

successively from the inner and outer surfaces of the

magnification. In other instruments, the relative positions

field lens. This system is common in prism binoculars.

of lenses not in the eyepiece are changed to effect

changes in magnification.

4-5