There are two camera controls for photographic exposure. One is the "f/stop" or diaphragm opening. Changing the f/stop changes the intensity of the light falling on the film. The other control is the shutter speed which controls the duration of the exposure.
The f/stops are in increments that result in double or half intensity on the film for each 1 stop change in f/stop. f/ 5.6 is twice the intensity of f/8 and half the intensity of f/4 etc. Shutter speeds are also in doubles and halves and are in fractions of a second. The steps on some shutters may appear to be in something other than doubles and halves ( 1/8 -> 1/15 for example) but they are sufficiently close to doubles and halves for practical purposes.
Exposure is a combination of intensity and time to achieve an image on the film. It is sometimes expressed as the formula E = i x t . The combination of f/stop and shutter speed is determined by the light conditions, the film speed and the intention of the photographer.
f/ 1.4 2 2.8 4 5.6 8 11 16 22
The scale of f/stops above represents decreasing intensity when read from left to right.
sec. 1/1000 500 250 125 60 30 15 8 4 2 1
The scale of shutter speeds above represents increasing time when read left to right. Put them together so that the time increased as the intensity decreases and you find that you have several options which will all result in the same exposure.
For example:
f/ 1.4 2 2.8 4 5.6 8 11 16 22 1/1000 500 250 125 60 30 15 8 4 2 1 sec.
If the correct exposure is determined to be 1/60 sec at f/5.6 then any combination of adjacent settings above is also the correct exposure. Since 1/125 sec. is half the time and f/4 is twice the intensity the resulting exposure is the same. 1/4 sec. at f/22 is also a correct exposure for this situation since you are increasing the time in the same proportion that you decrease the intensity.
By sliding the scales in relation to each other for different light levels we find similar options for each correct exposure:
1.4 2 2.8 4 5.6 8 11 16 22 1/1000 500 250 125 60 30 15 8 4 2 1 sec.
The scales above are adjusted to a correct exposure of 1/60 at f/2.8 (one quarter the amount of light in the previous example). It can easily be seen that correct exposure would also result from using 1/250th sec. at f/1.4, 1/8th sec. at f/8 or any other adjacent combination.
The choice of which combination to use is dependent upon the subject
matter and the effect desired. When photographing a moving subject you
will usually opt for a higher shutter speed to catch the action,
but when shooting a landscape or a closeup of a small object you may want
to put the camera on a tripod and use the smallest f/stop possible in order
to maximize the depth of field.
There is one more variable in exposure. It is the speed (sensitivity to light) of the film. This is expressed numerically as the ISO#. As the ISO # increases the sensitivity to light increases in direct proportion to the number; thus ISO 64 film is twice as sensitive to light as ISO 32 and roughly half as sensitive as ISO 125 film.
There are advantages/disadvantages to faster and slower films that I will cover in a separate lesson.
If you are using a light meter, be sure to set the meter to the correct ISO for the film you are using before taking a reading. If you have camera with a built in meter which reads DX codes, this will be set for you as long as you use DX coded film.
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