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THE FILM AND THE EXPOSURE
Exposure is the amount of light that is allowed to act or fall on the film. The aperture and the shutter settings control it. In order to control or set the correct exposure on the camera, knowledge of light sensitivity of the film and the brightness of the object should be known (i.e. lighting and tone)
The right size of the film should be selected which should fit in the camera e.g. 35mm wide that are packed in cassettes containing up to 36 exposures.
The film sensitivity is given in A.S.A (American standard association) or DIN number (for Europe) e.g. the film marked 400 ASA is twice as sensitive as that marked 200 ASA film.
These mean doubling the ASA number doubles the speed or its sensitivity. The faster the film, the less exposure needed to be given but the coarser or grainy your picture will be. i.e. higher ASA films produce coarse or grainy pictures compared to low ASA films, which give smooth and less grainy pictures, but however low ASA films require prolonged exposure. These is because a fast film gains its sensitivity by using larger grains of silver so the resulting negative film have a coarse granular pattern that can be clearly seen on enlargement.
Therefore, to decide on which film speed to use, you have to consider the lighting conditions and the subject matter of your picture. Films of 64,32or lower ASA numbers are slow and have very fine grains. They are the best choice is you want quality photographs with many details. However, they are slow and require lots of bright light. But films of 125 ASA still have fine grains; they are ideal for average subject surfaces outdoors and well-lit indoor subjects.
Faster films of over 400ASA show grains and are most ideal for very dim lighting or when grainy pictures are needed or when you want to freeze motion (i.e. to make fast moving object to appear to be stand still) while using fast shutter speeds.
NB most cameras nowadays have exposure meters and tables on their cameras, which help to determine the amount of exposure needed for a particular film