CAMERA FACTS
FILM

Okay so far we've looked at the bits of the camera concerned with composing, focusing and limiting the exposure to your image but how do you record it. Well it won't come as rocket science to appreciate that you need some way of capturing and recording your image and it is here film and digital cameras really differ

FILM

Most modern film cameras use cameras use a a photo sensitive emulsion mounted on a plastic type base (usually cellulose acetate) and coated in a scratch resistant material. For simplicities sake I'm going to talk about colour and black and white film that creates negatives when developed (film which gives rise to positive frames like slide film work in a very similar process)

The base simply acts as a support and as we've already seen during the early days of photography, glass was used as a base.

The real workhorse is the photo sensitive emulsion. This is made up of tiny crystal of of light reactive chemicals (like silver bromide) called "grains".

The size of these grains is important for two reasons.

Firstly one technique used to make film more rapidly reactive to light, is to increase the grain size and this explains why you can get high or slow speed films

The second reason is resolution of the film. Resolution represents the graininess of the film. With bigger crystals you will get a smaller number of reactive crystals. When you blow up photos you reach a point where clumps of grains on the film becomes discernible and the image appears grainy.

The importance in terms of films is that if you need to use high speed film (e.g. if shooting in poor light) graininess is more noticeable than on low speed high resolution film. Film speed are nowadays denoted by their ASA (American Standards Association) or ISO (International Standards Organisation) number. ISO & ASA speeds are essentially identical. Most general condition film is ISO/ASA 100 to 200. A slow film would be ISO/ASA 30 and a fast one ISO/ASA 1600

Now all these concepts probably make straight forward sense when considering Black and white film. But how do we get colour film you may well ask.

Simple. Most films are based on the same techniques except they use 3 layers of photographic emulsion are used (the so called tri-pack). Each layer reacts to one of the 3 primary colours - Blue, Green & Red. When crystals are activated instead of turning black they release a coloured dye.

Now how do you get from an exposed film to picture in you hand?

If you were mad enough to unroll an exposed film you would see no images and it would look like unexposed film. To get the image you must first develop it then permanently fix the image.

Although light sensitive crystals react when exposed to light the reaction is minimal. In the development process this reaction is chemically stimulated and areas exposed to light become darker and darker. You get a negative image (where what should be white appears opaque on the negative). Before the film becomes over developed it is place first into a solution to stop the developer's action before being put in a fixer solution which permanently stops this the photosensitivity of the film. Once the film is dry it is effectively the negatives you know.

To get prints you simply shine light through these negatives onto paper with a photosensitive emulsion (this too reacts to light in the same fashion as your negative film. So once processed dark objects (which are transparent on the negative) look the same on the prints.

(1) Figures From
Michael Freeman
The 35mm Handbook
revised 1st edition, 1989,
Publisher New Burlington Books
ISBN 0 906286 21 2

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