Figure 5-30. Eye relief-objective image outside erectors.

TM 9-258

view is thus inversely related to eye relief. If the eye

5-27. Factors Determining Eye Relief of Telescope.

relief is shortened and the aperture of the erectors is

a. Definition. Eye relief is the distance from the rear

increased accordingly, the field of view will be increased

surface of the eyelens to the plane of the exit pupil in

without requiring any increase in eyepiece aperture. If

which the eye must be positioned to view the full field

the size of the/eyepiece is increased and erectors are of

(para 2-34). Eye relief can be, and usually is, short in

corresponding aperture, the field of view will be

high-power observation instruments. In a binocular, it is

increased for the same eye relief.

quite short. Eye relief must be long in rifle scopes or

NOTE

gunsights where recoil is to be considered. Its location

will depend on the location of the real image

Coversion of angular field to linear field:

(0' in figs 5-29, 5-30 and 5-31) of the aperture stop

LF = 2aR, where LF equal Linear Field, R equals

(objective) formed by the erectors. The closer this is to

Range, "a" equals Angular True Field

the eyepiece the greater is the eye relief.

measured from the axis, and E equals 1

Radian

which

equals

57.3

(approximately).

Figure 5-29. Eye relief-symmetrical erectors with real images at focal points of errectors and image 0'

of objective between erectors.

Figure 5-30. Eye relief-objective image outside erectors.

Figure 5-31. Eye relief-collective lens in system.

b. Effects of Field Lenses. A field or collective

would make the eye relief too short in most cases for the

recoil. As mentioned in the note attached to paragraph

lens in the focal plane of the objective will shift this image

5-13b, the field lens in the eyepiece shortens eye relief

forward and shorten the eye relief (the stronger the lens

by shifting the plane of the exit pupil without affecting its

the farther the shift). For this reason, a collective lens

size.

usually is not used in a rifle scope.

5-16