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Chapter VII - Exposure control devices

What does exposure control do?

Each type of film has its own sensitivity, which is the same as the speed of this film. This sensitivity requires the appropriate amount of exposure to give a good and complete picture. The control device selects the appropriate group of light intensity and exposure period to suit the speed of the film. It is clear that the intensity of light is related to the amount of light emanating from the object to be photographed and the aperture of the lens. The shutter speed controls the exposure time

The basic control system was the one that included a phrase that said: take the picture only in bright sunlight, which was associated with the simplest types of photographic cameras. This is then followed by a series of weather-related icons that adjust the camera for cloudy or cloudy weather. This is a good system for anyone who wants to take pictures every day. As for the one who has a camera with multiple features and uses, he will find the equivalent of that system in a table that includes the exposure modes with the movie
Figure (29)

The correct exposure depends on the light and the speed of the film. The exposure in Figure A is given by any combination of shutter speed and aperture shown in Figure B. The exposure shown by the broken lines and position C will result in underexposure, while in position D we find overexposure, as it appears in position E.
All systems give first class exposures when used properly with natural or ordinary subjects. These systems can be misleading when things are too bright or too dark, and the lack of confidence in reading the exposure time scale rests on the experience of the person doing the imaging process. The first three systems contain an additional dynamic control. They allow the photographer to choose the combination of the speed and aperture values ​​that give the required field of sharpness in the image. In the fully automatic control system, we do not find this important creative feature in the control process. for the person

Do the different cells used for exposure meters create a real difference in the accuracy of the meter? The simple answer is that in all average conditions with the permission to use modern materials - even colored ones - after taking the readings from scales that have different cells, no noticeable difference will appear. However, not all images are taken from average subjects and inferred conditions, and herein lies the importance of the characteristics of cells of different scales

How do selenium cells (PV) work?

The first photoelectric exposure meters contained a selenium cell connected in a circuit with a galvanometer. The light falling on the cell causes it to generate a small charge of electricity, which in turn causes a needle connected to the galvanometer to move on a graduated scale to indicate the level of light. This was then converted into a combination of shutter speed and aperture. A selenium cell has a spectral response similar to that of the human eye, except that the cell needs to be large enough to generate sufficient power at light levels
Low to move the scale needle. Also, to maintain accuracy at these low levels, the needle must be precisely balanced. This makes selenium cell scales bulky, vulnerable to shocks and easily damaged

What other cells are used?

To secure durable and sensitive exposure meters and measuring devices for installation inside the camera, a standard cadmium sulfide cell has been invented. This cell is connected within a circuit with a small battery and provides a degree of electrical resistance that varies according to the intensity of the incident light. The brighter the light, the greater the resistance, and this is converted to indicate the exposure readings. Exposure meters with cadmium sulfide cells have few moving parts and are therefore reasonably shock resistant. It is also characterized by a very high sensitivity in the red region of the spectrum, which makes it ideal for accurate readings when lighting conditions are low, especially indoors.

Two other electronic cells have been developed, the silicon photodiode reacts very quickly - enough to measure electronic flash - and does not actually contain flash memory from read to read. The other cell is GaAs P (gallium arsenide phosphide).

All three electronic devices are able to achieve a high degree of accuracy under a wide range of different conditions in the areas of temperature / lighting and the subject to be photographed. Due to the rapid response of silicon diodes, they are used in devices that use readings directly from the surface of the film. It is therefore reasonable to choose a camera for the features it contains rather than the type of measurement cell in which it is used.