Figure 5-28. Linear true field.

TM 9-258

latter is the width of field at 1,000 meters. This is for

(fig 5-28). The apparent field of view is the angle at the

military instruments. Commercial distance is 100 yards

eye included by these two extreme principal rays.

Figure 5-27. Field limit.

Figure 5-28. Linear true field.

b. Determining True Field of Telescope. True

instruments). The field of view is limited by the optical

possibilities of the eyepiece, a consideration of the

field can be determined by sighting through the

aperture diameter, and the extent of the apparent field

instrument on some target at the edge of the field. The

for any given eye relief. Eye relief is the distance from

angular movement of the instrument required to shift the

the rear surface of the eyelens (para 4-4b ) to the plane

position of the target to the opposite edge of the field is

of the exit pupil (figs 2-34 and 5-25). The center of

the angular true field of the instrument (para 9-5).

c. Determining Apparent Field of Telescope.

rotation of the observer's eye should be situated in this

plane. For a given eye relief, apparent field (and thus

The apparent field of the instrument can be measured by

true field) is related directly to the aperture diameter of

repeating the above procedure while reversing the

the eyepiece.

For this reason, military telescopic

instrument. It will be found that the angle required will

gunsights frequently have huge eyepieces to provide a

approximate the true field multiplied by the power of the

large field at the long eye relief required in such an

instrument. Threrefore,

instrument. In a binocular or observation instrument

Apparent Field

designed to be used at any eye relief as short as 8

Power = -------------------- (a close approximation)

millimeters, an eyepiece of small aperture diameter will

True Field

provide a large field, when used in a system of such

5-26. Factors Determining the Field of View of

short eye relief.

Telescope.

b. Relation to Eye Relief. When eye relief is

a. Apertures. The diameter of the effective

reduced, the aperture of the eyepiece required to provide

aperture of entrance pupil (para 5-21) of the objective

the same angle is also reduced. The field of

lens is not a determining factor in any telescope except

in the Galilean system (unsuitable for sighting

5-15