أثر مسافة الرؤية في صحة منظور الصورة .. كتاب آلة التصوير

The effect of viewing distance on the validity of the image perspective

In order for the perspective of the photographic image to be completely similar to the natural perspective: the angle of view of the image of the object (1) must be exactly equal to the angle of view of the photographic lens. But where the angle of view of the photographic lens depends on its focal length, so the theory of perspective requires that the focal length of the lens be exactly equal to the distance of seeing the image in order to achieve the natural perspective (Fig. 126 p. 246). If the focal length is 15 cm, for example, the image must be viewed while it is 15 cm away from the eye, and if the viewing distance is 50 cm, then the perspective is correct - according to the previous rule - if the focal length of the lens is also 50 cm. !! However, it is very difficult to apply this theory in practice, for the following reasons:

1- It is impossible for us to always end up using a photographic lens whose focal length corresponds to the required viewing distance, because according to the previous rule, if we want to view the image, the distance must be at this distance.

2- It is very difficult to see the image at a small distance if a lens with a small focal length is used (as if the camera is a miniature and its lens has a medium focal length, for example, 1 cm), because according to the previous theory, we must look at the positive image at a distance. 5 cm is a natural perspective, which is difficult to apply in practice, as the human eye cannot see something that is only at this short distance from it, given that the Minimum Distance of Clear Vision for a healthy eye is approximately 25 cm. If we remove the image to a distance of 25 cm (which is five times the focal length of the lens), then the following results:

(a) That the perspective in the picture differs from the natural perspective (according to the theory also in order to obtain the aforementioned).

(b) The details appear small and cannot be clearly perceived. In order to avoid the previous difficulties that arose before the application of this theory, it was found that if the image is enlarged at a certain percentage, the details will appear clearer, and the image will acquire a natural perspective.

What is the percentage of magnification required for the perspective to be correct and normal?

This ratio is determined by the following two factors:

(a) The focal length of the lens used for imaging.

(b) The distance between the image and the human eye during vision (i.e. the viewing distance). The magnification ratio is estimated based on that according to the following law:

The distance between the image and the religion on the focal length of the lens = the required magnification ratio.

Example :
If the focal length of the lens is 125 cm, and it is desired to view the image at a distance of one and a half meters, for example, what is the required magnification ratio?

the solution :

Magnification ratio = the distance between the image and the eye on the focal length of the lens = 150 / 5, 12 = 12 times.

... To get a normal perspective the image must be enlarged twelve times.