Field of View.

TM 9-258

of interest is located, the eye turns in its socket until the

3-8. Field of View.

image falls upon the central portion, the fovea and

a. What may be seen by the eyes rotated in their

macula, and distinct vision is secured.

sockets without movement of the head is termed the field

c. The field of vision of both eyes includes portions

of view; what may be seen without movement of the eye

which are viewed by the right eye, the left eye, and both

is termed the field of fixation. The field of view of the

eyes (fig 3-6). The binocular field exists only in that

eyes is a horizontal extent of about 1600 and a vertical

portion of the field of view where the field of the separate

extent of about 70.

In comparison, the average

eyes overlap. This explains why stereovision or depth

telescope has a field of view of about 10.

perception is appreciated only in the center of the field of

b. The field of distant vision of immobile eyes is

view, it is not apparent, except by association, in the

extremely limited, distinct vision being limited to only a

outward portions of the field. The extreme outer areas of

very small central portion of the retina. Portions of the

the field of view serve in the perception of motion, light,

field focused on the remainder of the retina are seen

and shadow and form in two dimensions.

indistinctly and serve as a finder to locate objects of

interest. When an object

Figure 3-6. Field of view of the eyes .

(haze), light, and shadow; the difference in focus

3-9. Principles of Stereovision.

(accommodation of the eye) required for more than one

a. In a military sense, stereovision implies the

object in the field of view; and the relative apparent

ability to recognize the existence of differences in range

movement of distant objects as compared with that of

to objects by visual means only. It is an ability that can

near ones when the head is moved from left to right, all

be developed by training and practice. Certain fire

influence the estimate of distance.

control instruments are designed to greatly increase the

c. None of these means of depth perception are of

normal stereovision possessed by the individual.

value if its contributing factors are lacking. This is

b. There are various means requiring the use of but

difficult to realize completely in any practical case,

one eye which can be used to determine which of two

however, because one or more of these factors are

objects is the more distant.

present in almost all commonplace observations and are

(1) When one object is known to be larger than

subconsciously used by persons with two-eyed vision

another and both appear to be the same size, the larger

whenever possible to augment their stereovision.

object is known to be more distant.

Binocular vision can be investigated by placing two very

(2) If an object partly conceals another, the

small objects on a large table and then scanning them

second is recognized as being the farther away.

with the eyes

(3) Differences in the amount of atmosphere

3-8