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Page Title: Figure 5-30. Eye relief-objective image outside erectors.
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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
E
(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.
It
5-16

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