Section V. ADJUSTMENTS

TM 9-258

crosshair mounted on a reticle holder ring. If the eyelens

and the field lens are of the same power (focal length)

and the field lens lies in both the image plane and the

first focal plane of the eyelens, the image will be viewed

at infinity (without requiring accommodation by the eye).

NOTE

A field lens in the image plane shortens

eye relief by shifting the plane of the exit

pupil without affecting its size (para 5-

27b). The field lens also increases the

apparent field of the eyepiece.

Figure 5-18. Function of field lens in eyepiece.

Section V. ADJUSTMENTS

5-14.

Diopter Movement.

a. Zero Setting (Normal). In a well-adjusted

instrument the beams of light from each point in a distant

field or target will emerge from the eyepiece (of focal

length fe) as a beam of parallel rays. This is a zero

diopter setting (para 10-1), as diagramed in figure 5-19,

and will feed the eye with the same parallel light as that

received from distant points by the unaided eye. The

normal or emmetropic eye, therefore, can view the field

through the instrument without accommodation.

Figure 5-20. Minus setting for shortsighted eye

c. Plus Setting (Farsighted ). Farsightedness

or hypermetropia results in image formation in back of

the retina. Movement of the eyepiece away from the

recticle and the real image under observation, will cause

rays emerging from the eyepiece to coverge so that the

farsighted eye can focus the image clearly. This is a

plus diopter setting and is illustrated in figure 5-21.

Figure 5-19. Zero diopter setting.

b. Minus Setting (Shortsighted).

The

shortsighted or myopic eye will form an image in front of

the retina. Movement of the eyepiece toward the reticle

and the real image under observation, will cause

emergent rays from the eyepiece to diverge and correct

for the deficiency of the eye. This is a minus diopter

Figure 5-21. Plus setting for farsighted eye.

setting and is diagramed in figure 5-20.

d. Low-powered Instruments.

the

instrument is less than 4x, the eyepiece can be fixed

focus for satisfactory use. This means adjusting the

5-10