العوامل التي تحكم درجة التعريض ( Exposure ) -٢- كتاب فن التصوير الفوتوغرافي

Factors that control the degree of exposure (Exposure) -2-

To take a picture like the one in number 4, you have to increase the exposure a lot even though the sun is shining, because the trees block a lot of its light.
Suppose now that we put a wall to one side of the subject to be photographed, in this case we have blocked almost half of the light. If we put a wall on the other side, the subject is lit only by a narrow slice of the sky. Such subjects need more exposure in street snapshots as well, and this explains that many of them are underexposed (Photo No. 5).

Let us add to the two walls a roof, in such a way that the only light falling on our subject comes from the side, as a window in a house, in this way we will reduce the light. The size of the light in large quantity.
In this case, we may need to increase the exposure a hundred times or more.
Isn't it obvious, then, that shots taken under trees, near buildings, or at their entrances or inside buildings need a longer exposure period than those taken outside?
When estimating the degree of exposure, what matters to us is the amount of light falling on the subject itself and not the amount of light elsewhere.

In conclusion, we say: the category of the subject, whether it is a marine scene, a person in the shade, or a scene indoors, is what must be taken into account in order to determine the degree of exposure.

2- Intensity of light:

It is clear that the more intense the light, the lower the degree of exposure needed to bring about the required chemical reaction on the film.

Unfortunately, the human eye falls short of accurately estimating the intensity of light. The difference between the bright sun and a cloudless sky may be half the amount of light, yet the eye does not see a big difference between the two cases.
You might consider a well-lit room a draw
Lighting a sunny outdoor scene, although the difference between the two may be one to fifty or more. Here is the need to measure the intensity. of light accurately, and for this reason we direct the exposure meter.

3- Speed ​​of the lens:

We mention that the speed of a lens is determined by its f-value (f - value) and that a fast lens can open more than a slow lens. It is normal, then, that a fast lens with a large aperture receives more light, and requires a shorter exposure time. The issue of lens aperture has been dealt with previously, and here we need to say that. All photometers take into account the speed of the lens to determine the shutter speed to obtain a correct exposure.

4- Film speed or sensitivity:

Not all types of sensitive layers in films are affected by the same amount of light, as some are more sensitive and are affected by less light than others. For these sensitive or fast films, it is necessary to give them a lower exposure degree, either by narrowing the aperture or by increasing the shutter speed.

The photographer must know the speed of the film to be able to determine the correct exposure.
For example, to shoot a subject under a bright summer sun with a slow motion movie, we need a shutter speed of 1:100 of a second with an aperture of f/8, and to shoot the same subject under the same conditions with a medium speed movie, we need a shutter speed of 1:200 of a second with an aperture of F-16.

But if we use high-speed film, we need a shutter speed of 1:500 of a second with an aperture of F-22 or less.

Note: we call the degree of exposure: the shutter speed plus the aperture value.

We point out here that there are several systems for determining the speed of the film, and the system used in the United States of America is the ASA developed by the American Standards Association.

There is also a DIN system, and most modern photocopiers accept these two methods for setting the speed of the film.