الدلالات البصرية المؤدية إلى الشعور بالعمق في حالة النظر بالعين الواحدة ٢-a 

(Length and width) occupied by the shadows of the body, affecting our sense of the distance between the body and the wall (i.e. affecting the third dimension).

The proximity of areas of shadows with areas of light or high illumination has a stronger effect on the sensation of prominence, as the light rays fall on the prominent parts, casting shadows on what is directly adjacent to them from the less rose areas. The higher the ratio between the relative luminosity of the high-light areas and the shadows areas, i.e. the greater the contrast in the lighting, the greater the feeling of depth, while this feeling decreases if the ratio is equal, even if they are equal. This leads to a feeling of complete flatness, not depth.

If we discuss the previous rule on our sense of depth when we look at photographs, we find that this same factor also has an effect on feeling depth and prominence in photographs, even though they are only flat (Figures 122, 123).

The third indication:
Adjustment of the lens of the eye according to the distance of the body from it:

We knew before that before recording the image, it is necessary to measure the distance between the lens and the sensitive film according to the distance of the object from it: and the distance between the lens and the sensitive film decreases from it, and vice versa is also true. Likewise, the distance of the lens from the film is affected by its focal length, so this distance increases if the lens has a focal length. longer, and vice versa. We also learned that the means of adjusting the lens of the eye according to the distance of the visible object from it is done through the relaxation or contraction of involuntary muscles connected to the lens of the eye, so the degree of its curvature changes according to the distance of the visible object from it. Conversely, if the object is near, the muscles contract, so the degree of curvature of the lens increases, and its focal length decreases.

Here we say that the eye’s adaptation movement according to the distance of the body and what it entails of involuntary contraction or relaxation in the eye muscles is one of the indications that affect the individual subconsciously by feeling the proximity of the body or its distance from the eye.

Experiments have indicated that this indication does not help in perceiving the distance of objects that are more than six feet away if the sight is with only one eye and the other indications that help in perceiving the distance are not available. (1)

It is known in photography that this constraint also has an analogy, as if the lens is set at a distance equal to the first boundary (infinity) (i.e. at a point away from the lens by a distance = focal length x approximately 100), we will not need to re-adapt the distance of the film from the lens Mahazad The distance of the body from this point, so everything beyond it is considered in the rule of (infinity) as well, and the lens does not require a new setting.

Fourth indication:

Decreased depth of field as an effective factor in increasing the sense of the third dimension:

Depth of field affects the individual's ability to sense spatial depth. When the eye looks at nearby objects, it is seen as clearer than distant ones, and thus the disparity between the degree of clarity of near and distant objects has an effect on the sense of distance. This factor is also considered as one of the influencing indications of the feeling of the third dimension in the photographic image (Fig. 125 p. 243). In fact, photographic images are superior to the eye in its ability to control the depth of field, as, as we know, the human eye adapts to narrowness or wideness by means of involuntary muscles that result in The muscles of the iris narrow in the case of an increase in the amount of light, and widen in the case of a decrease in it, so controlling it is involuntary, but rather depends on the amount of light, unlike the aperture of the diaphragm, whose width can be increased (to control the depth of field) despite the increase in the amount of light, and it is not necessary at that time. Just need to increase the shutter speed.

And as long as it is easy to control the narrowness or the widening of the aperture of the rim of the lens, it is easy to control the depth of field, which itself is a factor that contributes to the extent of the sense of the third dimension. If we want to increase the “spatial depth” (i.e. increase the sense of the third dimension), the depth of field must decrease.

Fifth indication:

Obscuring parts of distant objects because others are located in front of them and are closer to the eye:

This factor does not require any explanation, as it only means a phenomenon that we all feel, which is the disappearance of some parts of distant bodies because others fall in front of them and are closer to the eye. The amount of the part of the distal body that is obscured depends on the following two factors:

(a) The relative distance between each object close to the eye and the one far from it. Therefore, it is not surprising that we find the head of a long-bodied person sitting in front of us in the cinema being sufficient to cover a large part of the projection screen.

(b) The relative size of each of the two bodies to the other. In sum, if there is a body that obscures one part of another, then the obscuring body is closer to the eye than the veiled one. This indication is the same as it is based on in the matter of photographs.

Sixth indication:

Changing the center of vision as a factor affecting the feeling of the third dimension:

If we look at a nearby object and a distant one, and then change the center of view (as if the head were moved to the right, for example), the closer object will appear to move in the opposite direction to the head movement, while the more distant object will appear to move in the same direction as the head movement. This phenomenon appears evident to us while we are on the train when you look at the telegraph poles close to the train, and they appear as if they were apparently moving in a direction opposite to the train's progress, while the distant trees seem as if they were moving in the same direction. .. And we infer from this example that changing the center of vision has an effect on the subconscious sense of the distance or proximity of objects. This indication has no analogue in the case of still photography, but it is available in moving cinematography (similar to eye movement).

Seventh indication:

Aerial Perspective:

It is usual, when looking at landscapes, to find a difference between the clarity of nearby objects and those that are very far away, whose color in nature tends to be white with blue as a result of the air separation between them and the eye. This indication is also similar in photographs, as the details of the closer objects appear more clear than the distant ones, which tend to turn white (Fig. 124).

The eighth indication:

Placement of objects in the image relative to the horizon line:

The horizon line in the image is used as a reference to indicate the distance or proximity of objects. The closer the body is to the horizon line, the more it is far from us. Even assuming that the horizon line does not appear in the image, the mere appearance of an object that occupies a high area in the image relative to another that occupies a low area in it is considered an indication of the distance of the first body from the second (Fig. depressions in it.