Retinal image size

to an object by comparing its apparent shape to its known shape. For

example, the greater the amount of detail you can recognize on an aircraft,

the closer you are to the aircraft.

(c) Vertical position in the field of vision. Objects tend to

be higher in the field of vision over distance. Therefore, you estimate the

distance to an object by its position in the field of vision. For example,

terrain features which are far away from the observer appear higher in the

field of vision than terrain features closer to the observer.

(2) Motion parallax: Motion parallax is the apparent motion of

stationary objects due to the motion of the observer. The speed with which

the object moves or whether it is stationary is dependent upon the distance

the observer is from the object. For example, fence posts close to the

observer will appear to be moving rapidly, trees at a greater distance will

appear to be moving more slowly, and mountains at even greater distances

will appear stationary. This cue to depth perception is considered

important because of its use during low-level flight.

(3) Retinal image size: The brain determines the distance to objects

by interpreting the size of the image focused on the retina. The brain is

able to determine the distance to an object by comparison of several

different factors. Those factors include the known size of the object,

increasing or decreasing size due to movement, terrestrial associations and

overlapping of contours.

(a) Known size of objects. The nearer an object is to the

observer the larger the retinal image is. By experience, the brain learns

to estimate the distance of familiar objects from the size of their retinal

image. A structure will subtend a specific angle on the retina based on the

distance from the observer. If the angle is small the observer knows the

distance is great. To do this the observer must know the size of the object

and have prior visual experience with it.

(b) Increasing and decreasing sizes of objects. If the retinal

image size increases, the object is becoming closer to the observer. If the

size decreases, the object is becoming farther from the observer. However,

if the size is constant, the object has remained a fixed distance from the

observer.

(c) Terrestrial associations. Comparing an object of unknown

size to an object of known size may be helpful in determining the relative

size of the unknown object and its apparent distance from the observer.

Ordinarily, the objects to be compared are judged to be at approximately the

same distance from the observer (Figure 9). For example, if you see an

aircraft (known size) in the vicinity of an airport (unknown size and

distance) you can judge the aircraft to be in the traffic pattern and that

the airfield is at approximately the same distance (determined from the

known size of the aircraft).

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