العلاقة بين سر عة الغالق وفتحة الديافراجم .. كتاب آلة التصوير

Chapter Eleven

Exposure

The relationship between shutter speed and aperture diaphragm

After the photographer adjusts the distance, i.e. adjusting the distance of the image from the lens to suit the distance of the object from it, the next step is to estimate the exposure factors. This expression is usually applied to the following factors:

(a) The focal number: it determines the intensity of the light that passes through the diaphragm aperture to the sensitive layer, and thus determines the intensity of the light image generated by the lens.

(b) Shutter speed: which determines the duration of the passage of light to the sensitive layer. The exposure must be sufficient for the photosensitive layer to be affected sufficiently to form the latent image. According to the foregoing, we find that:

Exposure = intensity of light (and it is controlled by the focal number) x time (and it is controlled by the shutter speed) i.e. Intensity x Time
= Exposure
It is possible, in any constant light conditions, to control the amount of light internal to the sensitive layer by means of the diaphragm aperture and the shutter speed (Fig. 172). By following one of two methods:

(a) Using a wide aperture diaphragm with a fast shutter speed. For example 4 f with a speed of 1 / 400 of a second ...

(b) Using a narrow aperture with a slow shutter speed. For example, 16 f with a speed of 1/25 of a second.

And if we look at the september places, we find that the amount of diaphragm that enters the sensitive layer is the same in both cases. In the first case, we used a wide aperture of the diaphragm with a large diaphragm speed, and in the second case, we compensated for the narrow aperture of the diaphragm by increasing the exposure time, i.e. increasing the duration of the passage of light to the flag. . The duration of the exposure increases by an amount equal to the ratio between the width of the first aperture: the width of the second aperture, and according to the previously mentioned rules by comparing one focal number with another, on pages 58 and 59 of this book (1) and according to the previous example, the exposure time must be increased based on the ratio between 16 f : 4 f
= 16 x 16 : 4 x 4
16:1
Therefore, in case B, we had to double the shutter speed by sixteen times, after the shutter speed was in the first case
1/400 of a second we made it 1/25 of a second in the second case, i.e. we compensated for what happened to the aperture of the Dia Farajm from its decrease to 16 f in the second case by increasing the exposure time sixteen times and making it 1/35 instead of 1/400 of a second. Here we may wonder: As long as the amount of light that enters the sensitive film is the same in both cases, as we mentioned above, when do you use the 4 f aperture with a shutter speed of 1/400 of a second, and when do you use the second case of 16 f with a shutter speed of 1/35 of a second? We explain it as follows:

There are two conflicting desires when photographing, and these two desires are:

(a) The desire to increase the shutter speed to record fast movements. (b) The desire to narrow the aperture of the diaphragm, in order to increase the depth of field. If we think that we should give the shutter speed the first consideration, then let us estimate it first in accordance with each of the focal length of the lens, the speed of the body, the distance of the body from the lens, and the direction of its movement. He mentioned it in this regard, but if what we mainly care about is obtaining a large depth of field, then we should start with estimating the diaphragm aperture in the first place, then we estimate the shutter speed that fits this aperture.

According to the previous example, we find that 4 f with a speed of 1/400 of a second is, for example, suitable for photographing fast movements, while 16 f is suitable for 1/25 when we want to increase the depth of field. It is obvious that we can also mediate between the two cases, and this is what we explain in the following table, which includes groups of (exposure factors), either of which are suitable for imaging in constant light conditions, as the amount of light that enters the sensitive layer is approximately equal in the case of choosing any of these groups.


According to the previous examples, we find that if we want to mediate between my desire to obtain a high shutter speed and increase the depth of field at the same time, we can choose, for example, any of the following groups.
50.6 at 1/200th of a second, or 8 f at 1/100th of a second. And if we assume, for example, that the previous table was suitable for certain light conditions, and that the amount of light was = K, then it is not suitable if these light conditions change, rather it must be modified, so assuming that it was suitable for the case of photography in bright daylight, for example, then it is not suitable for photography in bright daylight. In the shadows or in a closed place, as the exposure factors must be adjusted if the amount of light available changes during photography. Let us suppose that the power of light has been reduced by half, i.e. it becomes = 1/2 k, then we have the option to follow any of the following two methods:
(a) Either we fix the hole of the diaphragm and change the speed.
(b) Or fix the shutter speed and change the aperture of the diaphragm.
The following table shows the two factors of exposure when the amount of light decreases to half of what was appropriate in the previous table (i.e. when the amount of light = 1/2 k)

In the same way as the previous two examples, it is always possible to estimate the relationship between the aperture of the diaphragm and the appropriate speed in the event that the light strength is also increased to 2K or 4K, for example, or if the speed of the film sensitivity changes by increasing or decreasing. half,

There is a mistake that many people make when they believe that if the photography is under strong light (in sunlight), for example, then a narrow aperture must be used, and it is not permissible to use a wide aperture. It is very possible to use a wide lens aperture when photographing in strong sunlight, but this must be accompanied by a large increase in shutter speed (Fig. 173) and vice versa is also true. It is very permissible, even if the lighting is weak, to use a small aperture as well, but It is then required that the exposure time be very long, as it may reach several seconds or minutes if the light is very low.In Figure 174, an example of a case of photographing at night with the help of only the light of a street lamp.