زاوية قبول الضوء فى مقياس التعريض .. كتاب أسس التصوير الضوئي

Angle of acceptance of light in the exposure scale:

The electric cell is prepared in the exposure meter so that a protrusion from the body of the meter emerges around its sides, and its function is to determine the angle of acceptance of the light that affects the cell. If the exposure meter is directed to a certain area, this protrusion is what determines the rays that enter the cell. The angle confined between the top and bottom of two rays that allow them to enter the cell is called the vertical angle of acceptance, just as the angle confined between each of the right and left rays is called the horizontal angle of acceptance.

Accordingly, no matter how large the area from which the rays are reflected, the angles of acceptance: the vertical and the horizontal are the ones that determine the rays entering the cell. The surface of the electric cell that receives the light is usually rectangular, and rarely completely square or round. In the last two types, the horizontal and vertical angles of acceptance are equal in cells. With a rectangular plane, the two angles differ. As a general rule, it is observed in rectangular scales that the horizontal angle of acceptance is the largest, and the angle of acceptance in the exposure scale is related to the angle of view in the lens. To explain this relationship, we suppose that we are photographing a wide-area landscape with a camera with a medium focal length lens. In such a lens, the angle of view ranges around 560 degrees, and given that the place from which the light is measured is usually (but not always) in the location of the camera, if we use an exposure meter whose angle of acceptance of rays is more than 60, such as 80, for example, the electric cell in the meter receives rays from : Side areas that do not concern us in the image, thus increasing the percentage of rays that we do not need and that affected the scale by an increase estimated at 20: 60 i.e. + and thus the scale leads to a false reading that increased by one third. Practically, we find that an error in estimating the exposure by increasing or decreasing the amount of error. Simple and can be overlooked in some cases due to the readiness of sensitive films to resist such an error (Emulsion latitude), but this error may have its value if the angle of acceptance of the light in the scale differs greatly from the angle of view of the lens, as if the angle of acceptance is 80, for example, and the lens used is from Telephoto type and its angle of view is about 20

Then the area that is not desired to be photographed is confined and rays are reflected from it that affected the electric cell at an angle of 80 - 20 = 560, meaning that the error rate in reading the scale is 60: 20 = 3: 1, and this is a large percentage if we imagine increasing the exposure time by an error of three times, especially If that error is in a scientific research that needs accuracy, we still have to ask: What is the angle of acceptance in the exposure scales that we use? In fact, like the angles of view in different lenses. Some of them have a large angle of acceptance, others have a small angle, and most of them have a medium angle of acceptance that is commensurate with the angle of view of lenses with a normal focal length lens, meaning that the usual angle of acceptance is about 560, so it is suitable for the angle of view of lenses with a medium focal length. Some bodies concerned with the matter have differed in estimating the angle of acceptance in terms of standards. Exposure, for example, the ASA (The American Standard Association) considered that the angle of acceptance is the angle in which 60% of the amount of rays incident on the meter is confined and falls on the cell when the meter is directed to an attic roof. Lighting, distributed fairly
This is while the British Standard Institution (B) expressed the angle of acceptance as the angle that includes 89% of the amount of rays falling on the meter and entering the electric cell if the meter is directed at a surface illuminated by [fairly distributed illumination]
We conclude from the foregoing that there are no specific rules that determine the angle of acceptance in the scale. All that can be said is to try to get as close as possible between the angle of acceptance of the scale and the angle of view of the lens. Some factories have prepared light scales that are proportional to the angle of acceptance of the light. With telephoto lenses with sharp viewing angles, he made some experiments to find out the angle of acceptance in the scale he used .

Calculating the unknown angle of acceptance in the exposure meter:

We mentioned that calculating the angle of acceptance in the light meter has an important relationship with the angle of view of the lens, so the photographer must know both. With regard to the angle of view in the lens, this can be calculated from knowing the focal length of the lens and knowing the area of ​​the film it covers (and we have previously explained that, as for The angle of acceptance is easy to calculate and does not require anything other than a simple experiment, even though it is not infinite
Accuracy, however, gives us a practical, practical idea, and we show its steps in the following:

1- We shine a well-distributed light on a white surface of approximately one square meter, and put in the middle a black strip or a black circle with a diameter of about 40,400 cm, according to the shape of the electrical cell in the test meter. If the meter has a round cell, we put the black circle. If it has a rectangular or square cell, we put the black tape: the ratio of its length to its width is equal to the length and width of the cell. And it is required that the tape be completely black, and it is preferable for that to be velvet fabric. It absorbs a very large amount of radiation falling on it and reflects very little.

2- The light meter is directed towards the middle of the strip or the center of the circle and we gradually bring it closer to it. From the black strip or circle until he reaches a point where he stops permanently and no change occurs to him as he approaches the strip, and that is when it is. The black area has been included by the angle of acceptance in the scale.

3- From calculating the distance between the scale and the surface, and calculating the dimensions of the black surface, the angle of acceptance of the scale can be estimated. And if the scale has a rectangular cell, it is taken into account that both the horizontal and vertical angles of acceptance are measured.

​​​​​4 - The light acceptance in the scale can be expressed by numbers indicating either the field
Or numbers indicating the widening angle of acceptance. The “acceptance field” is estimated according to the ratio between the width of the black tape: the distance between the scale and the tape. If the width or diameter of the tape is one foot, and the distance between the tape and the point where the cursor stopped moving is 1.5 feet, then the ratio between them is 1:1.5, i.e. equal to 2. : 3 or about 0.66 approximately.

5- With the help of the following table, this ratio can be converted to know the angle of acceptance of the scale, and according to the previous example, the angle of acceptance in the scale is approximately 35. It was repeated here that the figure indicating the field of light acceptance was obtained from the ratio between:

The width of the black bar is the distance between the scale (when the cursor stopped moving) and the black bar