تأثير اتجاه قاعدة العدسة المتحركة في منظور الصورة .. كتاب أسس التصوير الضوئي .

Effect of moving lens base orientation on image perspective:

We have previously talked about the effect of the swinging back on the perspective of the image, and we learned that it is possible to fix the perspective by means of the swinging back, and the moving or swinging lens base as some call it cooperates with it.
In addition to the foregoing, the base of the moving lens has a major role in controlling the perspective as well, as by changing its direction and the direction of the machine, it is possible to exaggerate the perspective. This is something that photographers of engineering establishments may desire to exaggerate the magnitude or grandeur of the place, for example, or commercial advertisement photographers may be interested in it. The focal length of the camera lens plays a role in this, but there are Conditions in which a camera with a moving lens base must be used to achieve the desired effect from the image:

To make it easier for the anus, we show in Figure 59 five modes for a machine with a moving lens base prepared to photograph a building in front of it. As shown in the figure, the image we get in each mode differs from the other:

Mode A: shows the camera in its normal position facing the right wing of the building, so the image of this wing appeared in a rectangular shape and conforms to the real perspective in nature.

Mode B: shows the camera with its barrel tilted to the left side, while the base of the lens took a position in which it tends to the right side, so the image of the right wing appeared exaggerated in its perspective and increased from the right side, and at the same time the size of the left wing was small.

Mode H: shows the camera with its axis tilted to the right side, while the base of the lens moved to the left side, resulting in a large image of the left building and an exaggerated extension of its perspective in relation to the right wing of the building.


And situation D: shows the camera while it is in the same position previously mentioned in H, and only the base of the lens was repositioned to its normal place, which resulted in an increase in the size of the left wing and a decrease in the length of the right wing.

And situation E: the camera is in a position similar to case C, with an increase in the inclination of the axis of the camera to the right side, and an increase in the inclination of the base of the lens to the left side.. This resulted in a severe exaggeration between the perspective of both wings, so the left wing appeared very large in relation to the right wing.


Important notes regarding the use of cameras designed to fix perspective:

First - since we are forced to increase the extension of the bellows in one side of the swinging bale over the other when we want to fix the perspective, this results in the loss of sharpness of details (Unsharpness) on one side of the image - and in order to avoid the previous defect .. we must make a rule The moving lens (The Lens Standard). In flat parallel mode, the photosensitive film is moved by raising or lowering the lens. If he makes a drawing showing the vertical position of the position of each of the camera and the subject being photographed, it is seen that (a) the swinging back (b) the moving base of the lens (c) the flat object being photographed are all in a parallel shape (Figures 57, (58) and it is better to narrow the aperture of the diameter Farajm whenever possible To make the base of the moving lens in a position parallel to the surface of the photosensitive film. To remove part of the image to be recorded from the scope of the lens coverage.

Therefore, the factories that produce cameras designed to fix perspectives have always prepared the lens so that it covers an area larger than what is needed for the scale of the used machine.

We explain this in the following points:

A) _ If a camera measures 96 cm, for example, then the focal length of the lens must be approximately 10.8 cm in order for the lens to be considered a medium focal length. The area that this lens is able to cover is located at the focal length distance in a circle with a diameter of approximately 11 cm. Figure 60 Area No. (2).

b) _ If this machine has a swinging back and a swinging lens base, the diameter of the circle that the lens can cover should be greater than 11 cm, for example 15m, Figure 60, Area No. (5), including . The negative image (96 cm) is allowed to be included in the area covered by the lens. Details are sharp when the lens is raised or lowered.

c)_ To achieve that successor, the lens required for a camera whose scale has a swing-back and a movable lens mount such that the lens coverage power is adequate to a negative greater than 6. 9 x 9 cm is approximately the same as 9 12 cm, thus ensuring that the 6 x 9 cm negative image area is included in the area covered by the lens even though the base of the lens is moved up or down.