تعيين الرقم البؤرى المناسب لتحقيق عمق الميدان المطلوب .. كتاب آلة التصوير

Second: Determine the appropriate focal number to achieve the required depth of field:

After the photographer determines the middle point, he should also determine the appropriate f-number to achieve the desired depth of field. We will mention the following two equations, either of which is suitable for achieving this purpose:

(a) Focal number = (focal length) 2 x (after the middle point - after the nearest point) over the diameter of the mixing circle X after the middle point X after the nearest point

(b) Focal number = (focal length) 2 x (after the farthest point - after the middle point) over the diameter of the mixing circle x after the farthest point X after the middle point

And if we take the symbols previously explained on p. 2, it is possible to put the two previous equations in the following abbreviated form:

P = p2 x (x - s) on x x x x s. . . . . (1)

P = p 2 x ( b - x ) on x x b x x . . . . . (2)

Example :

What is the appropriate focal number to achieve a depth of field that starts at 6 meters from the lens and ends at 12 meters from it, if the focal length is 10 cm, and the camera measures 6 x 9 cm?

the solution :

First: We must determine the diameter of the permissible mixing circle.

A lens with a focal length of 10 cm is considered a focal length lens
Approximately average for a machine that measures 6 x 9 cm.

... The permissible mixing circle diameter = the average focal length on 1000 = 10 on 1000 = 1 on 100 cm.

Second: After the middle point
= 2 x 12 x 6 over 12 + 6 = 2 x 72 over 18 = 8 meters = 800 cm.

Third: Now we can determine the focal number according to any of the two aforementioned equations, No. 1 p. 215, No. 2 p. 217. This is as follows:

Applying the first equation, we get the following:

q = g 2 x ( x _ s ) over x x x x s

Focal number (f) = ( 10 x 10 ) x ( 800 - 600 ) over 1/100 x 800 x 600
= 100 x 100 x 200 divided by 800 x 600 = 200 divided by 48 = 1/6 of 4 = approximately 4f.

(2) By applying the second equation:

P = p 2 x (b - x) on x x b x x we ​​find the following:

Focal number (F) = (10 x 10) x (1200 x 800) over 1/100 x 1200 x 800
= 100 x 100 x 400 over 1200 x 800 = 400 over 96 = 1/6 4 = 4 f approximately.

Note: The two results that we obtained from applying the two equations for calculating the focal number (q) are completely consistent.