Photo
Exposure metering
In modern cameras it is measured with a high accuracy by electronic shutters. There is usually an exposure scale in manual mode and for metering the exposure. You’ll find several numbers here; they are of different length depending on the camera model, for example: 8 sec, 4 sec, 2 sec, 1 sec, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2000, 4000, 8000. These numbers indicate either whole seconds or fractions of a second. Good expensive cameras have a bulb position which indicates that the shutter is guided manually. It stays open as long as the release button is pressed. In Russian cameras this position is marked with «B» symbol.
Exposure can also be changed with the help of aperture. This word names a round opening inside the camera lens, which consists of several metal blades. The size of this light barrier can be altered by means of the ring placed on the lens carrier; most advanced cameras have special control units. Standard aperture scale consists of a row of successive numbers: 2; 2,8; 4; 5,6; 8; 11; 16; 22. Each number in this row is 1,4 time more or less than the neighbor number. This corresponds to the increase or decrease of light passing to the film exactly twofold at every turn of the aperture ring to one scale mark. The focal number indicates the amount of light passing to the film defined strictly for all camera lenses.
The exposure level can be altered over wide range with the help of exposure and aperture what completely covers any demand of a photographer. In different situations it’s sensible to use now a small exposure at big aperture value, opened “hole”, then, on the contrary, – a long exposure with the aperture closed. Open aperture agrees with the small numbers in the ring 2,8; 4. Closed aperture agrees with the big numbers 11; 16; 22.
Short exposures are used when it’s necessary to shoot a fast moving object in order to prevent blurring of the image. But the short exposure lets pass a little light to the film. To compensate this shortage of light, you must enlarge the “hole”, i.e. open the aperture, and vice versa.
Aperture affects not the exposure only. It also regulates the focusing depth. This depth is an important tool of a photographer. When focusing any object you’ll obtain a distinct image in the photo. But the neighbor objects, which were situated father or closer to the camera, also become distinct.
The distance between the object closest to the camera and the farthest one is called the focusing depth. The less is the aperture opening the more is the focusing depth. And vice versa: open aperture allows diminishing the focal depth so much that too aggressive background stops distracting the viewers attention from the focused object. Dark room may require both open aperture and long exposure.
Both the exposure and aperture scales are made in the same principle: every step leads to doubling of the exposure. That’s why the amount of light in the film will be equal both at exposure 1/30 with aperture value 8 and at exposure 1/125 with aperture value 4. Thus, these pairs are interchangeable. Scales of all exposure meters are based on the law of interchangeability.
How to choose position
In practice, photographer faces the problem of choosing such exposure and such combination of exposure and aperture which will contribute to the best reflection of all the colours of the shot object in the photo. Any object we shoot and its image, from the photographer’s point of view, represent a random mixture of spots with different brightness. If we measure the brightness of the darkest spot of the object and the lightest one which we want to make distinct in the pictures, the difference of brightness mustn’t exceed 100. Black-and-white films can reproduce the difference twice as much. Colour negative films reproduce a smaller difference in brightness. In practical photography, only a mistake in exposure choice can prevent you from obtaining a picture with correctly reproduced details, both in light and dark parts of the image.
Not to make mistakes you must remember the scale invented by American photographer Ansel Adams. He divided all existent brightness into ten zones: from the darkest to the lightest.
6. Adams scale
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