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Film

by Peter Scoones

Reproduced from in focus 10 (June 1985)

There are several different types of film available in 35 mm or larger format, and each type has advantages and disadvantages. The choice within each type is greater for 35 mm than large format.

Black and White Negative Film

Black and White negative film is available in varying speeds. These may be classified as slow (up to 50ASA), medium (50 - 200ASA), fast (200 - 400ASA) and ultra fast (over 400ASA). Slow film has fine grain (and hence high definition), high contrast but low latitude. (Latitude is the amount by which a film can be over or underexposed and still produce a reasonable image). Fast film has a coarser grain (and hence lower definition), lower contrast, but more latitude. Film speed is determined by the minimum exposure required to change the silver in the film emulsion to a latent image. This minimum exposure is the threshold, exceed it and the film is exposed. Fast films are hypersensitised either chemically or by flashing a bright light of known intensity for a brief known period to just below the threshold i.e. not quite enough to produce a latent image.

Exposure is controlled by time and light intensity. There is generally a linear relationship between them i.e. the more light the less exposure and vice versa, although this relationship breaks down for very short or very long exposures. The exposure controls density.

Development changes the silver in the emulsion to metallic grains, the faster the film the larger the grains. Development controls contrast. Film speed Is not affected by development. When you uprate or push a film, the Film speed stays the same, but the contrast of the resulting image changes. So if you uprate say a 100ASA film by 1 stop to 200ASA. you are actually underexposing that film by 1 stop. When developing that film you increase the development time (by 33% in this case) mid consequently increase the contrast. Sow underwater photographers deliberately underexpose their films slightly to increase density and then over develope slightly to increase contract.

Colour Reversal Film

Colour reversal or slide film is available at similar speeds to Black and White negative film. but only has a latitude of about 1/2 stop. There are many types available and all strive for neutral balance, though few acheive it. The emulsions used have a tendency to favour certain colour shifts i.e. the old Ektachrome 64 was always regarded as slightly blue. The processing and conditions affect each film type differently. They can be pushed or uprated but care should be taken. Kodachrome is the moat stable and has the highest definition. It is essentially a black and white film. the colour being introduced at the processing stage. Professional film differs from others in that it is intended to be stored under cold conditions prior to use and then processed immediately. Other types of film have a longer shelf life and although they degrade, do so much slower than professional films.

Colour Negative Film

Colour negative film is available in speeds from 100ASA to 1600ASA. It has a very much greater latitude than either black and white negative or colour reveral film, about 5 stops. As with colour reversal film there are many types available, All are very similar and there is little to choose between them. Fuji has the most contrast and the most critical exposure. Kodak is middling and Agfa is the safest. Colour negative film can be used for colour prints or transparencies and black and white prints.

Special Films

In addition to the standard colour negative, colour reversal and black and white films, special films are available for duplicating and printing and for infra red photography. Unlike standard films you will need to go to a specialist suppliers in order to buy them.

Reproduced from in focus 10. June, 1985


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