Binocular (General Use)

TM 9-258

b. The power or magnification of the binocular,

eyepiece can be focused independently of the other by

turning the diopter scale in a plus or minus direction.

like that of the monocular telescope, depends upon the

The scale, which is calibrated in diopters, indicates the

focal lengths of the objective and eyepiece groups. The

correction required for the corresponding eye. Once the

true field of view depends upon the design of the lenses

proper setting for diopter adjustment has been

and the power. The brightness of image depends upon

determined for each eye, the setting may be applied

the size of the objective. The amount of the exit pupil that

immediately on future use.

can be used depends upon the size of the pupil of the

eye of the observer which varies from about 2 millimeters

for very brilliant illumination to about 4 or 5 millimeters in

8-19.

Binocular (General Use).

daytime, and about 8 millimeters for very faint

a. The binocular illustrated in figure 8-21 is

illumination. The design of an instrument of this type

typical of those designed for use as a general purpose

determines its suitability for a specific purpose. A

instrument by all services for observation and

binocular with large objectives and exit pupils is generally

approximate measurement of small angles. It has a

better suited for observation at night and other conditions

magnification of 6 power, the objective diameter is 30

of poor visibility. A binocular of this type is often referred

millimeters, and the field of view is 80 30 minutes. Each

to as a "night glass."

of. the prism offset-type telescopes has an achromatic

c. Binoculars usually are designated by the

objective and a Kellner-type eyepiece with a compound

power of magnification and the diameter of the

eyelens and a plano-convex field lens. A porro prism

objectives. Thus, a 6 x 30 binocular magnifies 6

erecting system is employed in each telescope. The

diameters and has objectives which are 30 millimeters in

eyepieces are individually adjustable from plus 4 to

diameter. This designation usually is stamped on the

minus 4 diopters to meet eyesight variations. Graduated

instrument.

diopter scales permit prefocusing of both eyepieces.

d. Binoculars permit the use of and increase

The interpupillary distance adjustment also is provided

the radius of stereoscopic vision. The observer views

with graduations to permit presetting.

the object from the two objectives which are more widely

b. A reticle is included in the optical system for

separated than his eyes, while the magnification

the left eye (I, fig 4-25). The binocular is equipped with a

provided by the instrument increases his range of vision.

filter and a carrying strap. This model has improved

For example, if the distance between the lines of sight of

waterproofing.

his eyes is doubled by the use of prism binoculars and a

6-power instrument is used, the radius of stereovision is

8-20.

Battery Commander's Periscope.

increased from a normal of approximately 500 meters to

a. The

battery

commander's

periscope

approximately 6,000 meters (500 meters times 2 times

illustrated in figure 8-22 is a binocular instrument which

6).

consists of two periscope-type telescopes joined at the

e. The binocular hinge is equipped with a scale

top by a hinge mechanism, to permit adjustment of the

which indicates in millimeters the interpupillary distance

distance between the eyepieces. The eyepiece being

(distance between the pupils of the eyes). When the

about 12 inches below the line of sight. The BC

proper setting for the observer has been determined, any

periscope is not hinged at the bottom (as were earlier

binocular may be adjusted at once for the correct

model BC's periscopes) and is typical of those designed

interpupillary distance.

to be used only for periscopic observation, with the

f.. All of the binoculars covered in this manual

telescopes in a vertical position.

are constructed for separate focusing; that is, each

8-18