( شرح الجدولين رقم ٥١،٥٠ ) مقدار نقص أو زيادة التعريض حين استخدام عدسة إضافية

(Explanation of tables No. 51 and 50)

The amount of under or over exposure when using an additional lens

First: In the case of using the positive additional lens (Table No. 50 p. 108) when a positive additional lens is placed in front of the original photographic lens, the focal length of the two lenses must be less than the focal length of the original lens alone, which results from the other side - that the capacity of the two lenses increases ( together) to receive the light than what would have happened if the additional positive lens had not been placed in front of the original lens, and so we see that we must work then to reduce the exposure by a certain amount that depends on each of the following factors:

(a) The focal length of the original lens (fixed with the camera)

. (B) additional positive.

Table No. 50 p. 108 shows us the amount of exposure required for the size of the camera.

Example: When filming titles for a movie, I saw that an original lens with a focal length was used, and the focal number estimated by the exposure meter was 8 f, then I placed in front of the original lens an additional positive one whose strength is + 4 diopters, meaning that its focal length = 25 cm, so what is the amount of the decrease in the required exposure Then, what is the new focal number that must be set in the original lens after placing the additional positive lens in front of the original lens?

Answer: From Table No. 108, we conclude that the amount of the nursing deficiency = 1 stop, and by reference to Table No. the focal number F.No that decreases by - 1 stop from 8 a is O.1. 34 p. 63 We conclude based on

Second:
If an additional negative lens is used
(Table No. 51 p. 109).

Contrary to what we have presented, it may sometimes require that an additional negative lens be placed in front of the original fixed lens in the camera, not for close-up shooting conditions, but rather to increase the focal length of the original lens, because the additional negative lens - if placed in front of the original lens - will It increases the focal length of the two lenses together over the focal length of the original lens alone. As for the additional positive lens, it works to shorten the focal length, and it follows from that that the exposure must be increased when using an additional negative lens. Table No. (51 p. 109) guides us about the amount of this the increase .

Example: Assuming that the focal length of the original lens = 10 cm, and the focal number dictated by the nursing scale in accordance with the light conditions = 4 8, and the focal length of the additional lens = 38 hm, i.e. its power = -2.6 dew Peter). What is the required increase in nursing after placing the additional negative lens in front of the original lens, and what is the new focal number?

Answer: From Table No. 51 p. 109, we conclude that the amount of increase in nursing = 1 endowment. This means that the new f-number = 5.6 f - instead of 8 f.