العلاقة بين البعد البؤري للعدسة ومساحة الصورة السلبية .. كتاب آلة التصوير

The relationship between the focal length of the lens and the negative image area:

Where it is imperative to place the sensitive film so that it is within the optically superior zone (area No. 2, Figure 63) covered by the lens, and where the diameter of this circle is completely or almost equal to the chord that connects two opposite corners of the negative space, and given that the length of this chord is It depends to a large extent on the focal length of the lens, so the length of this chord is taken as a standard criterion for determining the characteristic of the lens (ie if it is long, short, or medium focal length).

A law known as the Law of Diagonal has been developed for this, and it states that a lens is considered to have an average focal length if its focal length is equal in length to the chord that connects two opposite corners of the area of ​​a sharp, evenly luminous image located on its focal plane. (Fig. 67, 68, 69 ).

Accordingly, the lens is considered to have a long focal length if its focal length is greater than the length of this string, and it is considered to have a short focal length if its length is less than this string. Example :

If we suppose that the area AB C D (in Figure 69) represents the image area in a camera measuring 9.6 cm and the focal length of the lens is 13 cm, is the lens considered to have a long, medium or short focal length for this machine?

To answer this, rvh must be defined as the length of the hypotenuse ?

( a c ) 2 = ( a d ) 2 + ( d c ) 2

0 ( a c ) 2 = 9 x 9 + 6 x 6
= 81 + 36 = 82, 10 cm

That is, in order for the lens to be considered an average focal length if used with any camera measuring 6 x 9 cm, its focal length must be 10982 cm; If its focal length is greater than that, we consider it to have a long focal length, and if it is less than this serious, we consider it to have a short focal length. Thus, the lens whose focal length is 13 cm (which is mentioned in the previous example) is a long focal length.

Based on the foregoing, a lens whose focal length = 8, 10 cm can be considered an average focal length if it is used on any camera whose size is 6 x 9 cm, while if it can be installed on a camera whose size = 6 x 6 cm, then it is considered A long focal length lens for this machine, and if it can be installed on a photograph measuring 9 x 12 cm, then it is considered a short focal length lens.
We see in the following table a statement of the dimensions of the different images and the length of the chord in each image.

Actual viewing angle report:

We saw in the foregoing that the degree of correction of the lens and the extent of its ability to form an image with sharp details constitutes an important element in determining the extent of the narrow or wide angle of view of the lens (in addition to the main factor which is its focal length). We follow the following method (Fig. 70):

(a) We draw a line like Ab whose length is equal to the diameter of the widest circle in which the details of the image are completely sharp and the intensity of illumination is equal. It is clear that this diameter should be equal to the chord that connects two opposite angles in the largest area (whether square or rectangular) located within this circle (and this area is the area that represents the area of ​​the film or the sensitive plate in the camera).

(B) A vertical line, such as a grandfather, is drawn exactly in the middle of Ab, so that the length of this line is exactly equal to the length of the focal length of the lens.

(C) By connecting the two lines A and D, we find that they contain an angle that is itself equal to the actual viewing angle.