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PERSPECTIVE PERSPECTIVE

The effect of the focal length of the lens on the correctness of the image perspective

Suppose we photographed a house as shown in Figure (47).
And they used a single camera with three lenses, the first with a long distance, the second with a medium, and the third with a short focal length: as it is, you notice the following:
A _ Between the lengths of the two sides is 1 200 equal proportions in their distance
Focal length.

B _ The ratio between the lengths of the two sides increases when using a lens
The focal length is average.

A- The ratio becomes large between the lengths of the two sides when a lens with a short focal length is used

If we go back to nature and look at the house. We found that image (1) is the closest of the three images to the normal perspective. While we find a great exaggeration in the ratios of lengths when photographing with a lens with a short focal length, as in the picture (c), and the error in the image’s perspective increases when the camera approaches the object to be photographed, meaning that large-sized nearby objects appear! Too much for that lens that appears too small. (fig. 48)

The correct distance to see the image:

The photograph is supposed to be truthful in conveying the subjects that were photographed: in order to reach this goal, several factors play roles in order for the photograph to be in conformity with reality. The aperture of the lens diaphragm has a role in that, the shutter speed has a role, and the steps that follow showing the negative and positive images have roles.

But one of the main factors that we often neglect is that the image must be seen at a certain distance from the eye in order to see the correct perspective. And to explain this are the same facts on which he relies to obtain a naturalistic perspective. If we look at a body, an infinite number of light rays will be reflected from that body that the eye can sense. Let us suppose that this body is a person. Standing in front of the eye, the observer sees that the rays reflected from the top of his head and from his feet form an angle of vision with the eye.. This angle narrows or becomes sharp according to the distance between the eye and this object. The closer we get to the object, we notice that the angle of view becomes more open, so our sense of the proximity of the distance increases, just as our sense of its large size increases according to (b) the opposite increases. Figure 49. If we photograph the body of this person in such a way as to obtain a perspective similar to the perspective we see in nature, and the photograph faithfully expresses the sense of distance and the sense of correct proportions of near and far objects to each other, the following points must be noted:

(1) The photographic lens must have an angle of view equal to the angle of vision when viewing the object in nature.
(2) Because this angle of view is related to the distance between the eye and the object being viewed, then we must look at the image at the distance at which the angle is the reflected rays. The angle confined between the top of the head and the foot is the same angle that the photography lens confined between each of the top of the head and the foot in nature. And given that the focal length of the lens is what determines its angle of view.., so it was. Focal length is an important factor in the validity of the image perspective. If these two conditions are met, the conditions for the eye to see the image become the same conditions in which the photographic lens recorded (or saw) what was in front of it in nature, then the perspective is natural. (fig. 51)

But what is the focal length of the lens that must be used in order to obtain an image that matches the natural perspective?

The theory of perspective requires that the focal length of the lens be equal to the distance between the image and the eye that is looking at it. If the focal length of the lens is 15 cm. The picture must be viewed at 15 .
And if the vision distance is 50 cm. According to this rule, the perspective is correct if the focal length of the lens is its length. 50 .
This is what we wanted to apply this theory in practice: We found the following: It is impossible to always obtain a photographic lens whose focal length corresponds to the required vision distance of 50 cm, for example (1)

Examples (2) It is very difficult to see the image at a small distance in the case of using . A lens whose focal length is small, shaped like (50), as if the camera used was a Miniature Camera, and the focal length of its lens is 1 cm. In application of the previous theory, we must look at the positive image at a distance of 1 cm in order to get a natural perspective of 1, which is difficult to apply. Practically, since it is impossible for the human eye to see something at such a small distance - as it is known that the vision distance for the healthy eye is approximately 25 cm - so if we remove the image to a distance of 25 times the five times the focal length of the used lens

(a) The difference of perspective in the image from nature is a time for the aforementioned theory

b) That the small details appear and cannot be perceived clearly, and in order to avoid the difficulties in applying the theory, we can enlarge the image, so the details appear clearer, and the image acquires a natural perspective (Fig. 51)