التعارض بين حالتى استخدام الظهر المتأرجح لزيادة عمق الميـدان و استخدامه ١-a .

The conflict between the two cases of using the swinging back to increase the depth of the field and using it to keep the ribs parallel or perpendicular to the ground:

If we go back to some of what we mentioned in the previous pages, we would find a contradiction between the two means of achieving two desires that we sometimes want to combine, and these two desires are:

(a) Increasing the depth of field, which is a condition that calls for increasing the extension of one side of the swinging back to suit the dimension of the side closest to the object being photographed, with: its extension on the other side (which is position A in Figure 137).

(b) Keeping the vertical sides on the ground or the parallel lines in nature also vertical or parallel in the image, and in order to achieve this purpose, we require that the flat of the sensitive film be parallel to the vertical, parallel sides of the subject to be photographed (which is position B in Figure 137).

Thus, we find a contradiction between the swinging back positions in the two cases. However, it is possible to overcome this contradiction, by tilting the top of the lens towards the front and the bottom towards the back without moving the swinging back (Fig. This position is considered ideal if we wish to combine the depth of field and the perpendicularity of the ribs to the ground, as it is thus the only correct position when photographing engineering facilities, showing them and what lies in front of them in the foreground of the image, all in sharp detail (Fig. 139).

Shooting from a side position while maintaining the horizontal parallelism of the sides:

Correct photographic rules dictate that the camera must be placed facing the surfaces to be photographed, so that the proportions between its dimensions (length and width) are correct in the photograph. Therefore, the optical axis of the lens must be completely perpendicular to the middle of the plane that we want to photograph (case 1, Figure 143). However, it may not be possible to apply the previous rule sometimes due to the nature of the plane that we want to photograph as if it were shining like a mirror, for example, or due to the nature of its position in the room or place. Such as the case of photographing an oil painting that cannot be moved, and in front of it there is a barrier obscuring part of it (Fig. 145). Therefore, we may think, then, of placing the camera in a side position, which is also unpopular, as it results in the sides not being parallel in the image, or the length-to-width ratio differing from what it is in nature (Fig. in the picture. (Case 2 in Figure 143). For the aforementioned reasons, we have to use a camera with a swinging back, with an increase in the extension of one side of the bellows, so that the surface of the sensitive film is completely parallel to the surface of the body. The placement of the camera is arranged as shown in Case No. 3 in Figure 143 on page 264.

This method is also valid when photographing cube-shaped bodies or parallelograms when it is desired to combine the images of the front and side planes while maintaining the parallelism of the sides (Figures 146, 147). In situation No. 2, Figure 146, and it is a situation that results in the appearance of the two horizontal sides on the opposite side, tending to converge on one side of the image, which is something we may not want if we want the image to be the basis for an accurate measurement or a specific technical study. However, this problem can be overcome by using a swing-back machine with an increase in the extension of one side of the bellows so that the flat of the sensitive film is completely parallel to the surface of the retracting side (the third position in Figure 146).