( شرح الجدولين رقم ٥٦ ، ٥٧ ) سرعة حساسية الفلم المعدلة في مقياس التعريض .

(Explanation of tables No. 56 and 57)

Adjusted film sensitivity speed in the exposure scale to suit the filter factor

1- It is known that the required first step when starting to use the Exposure Meter is that it must be set to suit the sensitivity of the film. (Determined in DIN, Scheiner, .... etc.)

2- It is also known that the use of light filters results in the amount of light that permeates the filter and penetrates the film to be less than that which falls on the filter, and to compensate for this deficiency, the matter requires that the exposure be increased by a certain amount that is known as the filter factor.

Table No. 56 p. 120 shows us the speed of the film sensitivity (estimated in American units.A.S.A.) that must be set in the exposure meter if we want the exposure meter to be the direct means for estimating each of the diaphragm apertures and shutter speed that are suitable for each of the light conditions and the filter factor together. . As for Table No. 57 p. 121, it differs from Table No. 56 in that the film’s sensitivity speed was estimated in DIN units.

And such tables are suitable in cases that require recording multiple images with the use of a specific filter, instead of reading first in the nursing scale each of. The diaphragm and the speed required if there is no optical filter, then the nursing factors, namely the aperture and the speed, are recalculated before each shot in a way that suits the filter factor. It has found a shortcut to the steps and the speed of work. It is possible to adjust the sensitivity of the film in the exposure scale to an extent commensurate with the filter factor. The reading that we take from the scale indicates what must be done directly without any new steps.

Example 1: If the film sensitivity speed is 120..A.S, and the “filter factor” = 2.5, what is the modified film sensitivity speed (that is, that must be set) in the nursing scale to suit the use case of this filter?

Answer: Referring to Table No. 56 p. 120, we conclude that the exposure should be estimated as if the film's sensitivity speed was: A.S.A. o.

Example 2: If the film sensitivity speed is: DIN = 23 and the “filter factor” = e, then what is the adjusted film sensitivity speed in the exposure scale that is suitable for the case of using this filter?

Answer: Referring to Table No. 57 on p. 121, we conclude that it is necessary to estimate the new exposure as if the film’s sensitivity speed = DIN16.