Section IV. DEFECTS AND LIMITATIONS OF VISION

TM 9-258

i. Angles of convergence become smaller and

be developed by practice and training which aids

recognition.

The smallest difference between

the difference between them becomes more difficult to

convergence anges that an observer is able to

discern as the objects are farther removed from an

distinguish is known as his minimum discernible

observer or as their distance from one another is

difference of convergence angles.

decreased. This difference is known as the discernible

difference of convergence angles (B, fig 3-9) and is

measured in fractions of minutes and seconds of arc.

l. The minimum difference that can be

This provides a means of gaging the ability of observers

discerned between two angles of convergence depends

to see stereoscopically.

upon the vision of the individual observer, his state of

training, and other general conditions such as visibility. A

j. The sense of depth perception or

well-trained observer should consistently discern an

stereoscopic vision for the unaided eye is effective, at

average difference of 12 seconds of arc and at times,

most to 500 yards. In using a binocular or range finder,

under excellent observing conditions, this difference may

the lines of sight for the two eyes are further separated

be reduced to as low as 4 seconds of arc for a series of

thus increasing stereoscopic vision.

observations.

k. Stereoacuity (or stereopsis) is sharpness of

The average untrained observer should be able to

sight in three dimensions. It is the ability to gage

distinguish a minimum difference of 30 seconds of arc

distance by perception of the smallest discernible

between two angles of convergence under normal

difference of convergence angles. This ability may

conditions of visibility.

Section IV. DEFECTS AND LIMITATIONS OF VISION

Old-sightedness (presbyopia)

3-10. General Eye Defects.

Astigmatism

Colorblindness (dichromatism)

a. Classification of Eye Defects. Common eye

Night blindness

defects fall into two classifications: those pertaining to

Muscular imbalance (heterophoria)

sight itself and those which are the result of muscular

Double vision (diplopia)

disorder. Names ending in "ic" signify the condition while

Squint, cross-eye, wall eye (strabismus)

"ia" signifies the name of the disorder. Likewise, the

c. Nearsightedness and Farsightedness. The

"tropias" derived from the Greek "op" meaning sight,

size of the eyeball, the curvature of the cornea, and the

signify optical or sight abnormalities while the "phorias,"

focal length of the compound elements are not always

from the Greek "phora" meaning movement, signify

matched. If the lens is too convex or the retina too far

muscular disorders. The normal eye is said to be

back, the eye is nearsighted or myopic. This condition is

emmetropic; the person enjoys emmetropia.

The

corrected by negative or digerging spectacles (fig 3-10).

abnormal eye is ametropic; the person suffers from

If the lens focal length is too long or the retina too close,

ametropia.

the eye is farsighted or hypermetropic. This condition is

b. Common Eye Defects. The common eye

corrected by positive of converging spectacles (fig 3-11).

defects are listed below. Explanation may be found in

the glossary.

Nearsightedness (myopia) Farsightedness

(hypermetropia

hyperopia)

Figure 3-10. Nearsightedness (myopia)

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