حساب زاوية القبول المجهولة فى مقياس التعريض للضوء .. كتاب آلة التصوير

Calculating the unknown angle of acceptance in the exposure meter:

Since the angle of acceptance in the exposure scale is related to the angle of view of the lens, the photographer must know their understanding. We have previously explained how to calculate the angle of view in the lens from knowing its focal length and the area of ​​the film (refer to p. 106). As for the angle of acceptance, its calculation is also easy and requires only a simple experiment. Although it is not infinitely accurate, it gives us an approximate practical idea. Here are its steps:

1- A white surface (its area is approximately one square meter) is lit so that the lighting is completely evenly distributed over it. And we put in the middle of it a black tape or a black circle with a diameter of about 40 cm, according to the shape of the photoelectric cell in the scale. If the scale has a round cell, we put the black circle (Fig. 184), and if its cell is rectangular or square, we put a black strip whose length to width ratio is equal to the length and width of the cell. It is required that the tape be completely black, and therefore the velvet fabric is preferred, as it almost absorbs all the rays falling on it and reflects only very few of them.

2- The scale is directed towards the middle of the strip or the center of the black circle, and we move the scale towards it and gradually bring it closer to it. Here we notice that the indicator of the exposure meter deviates gradually towards zero, indicating a decrease in the intensity of illumination as the meter approaches the tape or the black circle, even if the meter reaches a certain distance from the tape, we find that the indicator has been permanently fixed, and it will not be affected no matter how close the meter is to the tape. Here the black region has been included in the angle of acceptance in the scale.

3- From calculating the distance between the scale and the black plane, and from calculating the length and width of the black plane, we can estimate the angle of acceptance in the scale (1).

4- The following percentage is calculated:
For example, if the width or diameter of the tape was one foot, and the distance between the tape and the point where the cursor stopped moving = 1.5 feet, then the ratio between them is 1:1.5, i.e. equal to 2:3, i.e. about 0.66 approximately.

E - With the help of the following table, it is possible to know the angle of acceptance of the scale that is appropriate to the previous ratio, and according to the previous example, the angle of acceptance in the scale is 35, and it is also possible to calculate the angle of acceptance according to what came in (Fig. 185).