Film

The film is a flexible transparent material coated on one side with a layer of gelatin. On black & white film, evenly distributed within the gelatin layer of 'emulsion' are billions of microscopic crystals called silver salts or halides. These particles are light sensitive, which means that when they come into contact with light they undergo a rapid physical change. It is upon this property that the whole photographic process is founded.
The structure of colour film is more complex than that of black & white but the same principle is utilised. In effect, three latent images are formed instead of a single one, each on a separate layer of emulsion which responds selectively to light in different parts of the spectrum. These different colour sensitive layers (blue, green and red) are all coated on the same film base.

Film speed is an important quality of both colour and black & white film. A fast film is one that is highly sensitive to light and therefore takes up an image very quickly. A slow film is less sensitive and needs a longer period of exposure to light. The difference is based on the size of the crystals in the emulsions. Large crystals react more readily than small ones, therefore fast films have large crystals and slow films have small crystals. These crystals can be seen on the printed enlargements as grain. Therefore prints from faster films show more grain than that of slow films.

Different scales are used to express film speed. The most common is ASA which stands for American Standards Association.

Typical film available are :

                    50asa,  100asa,  200asa,  400asa, 800asa and 1600asa.

You can see that the asa number doubles from one to the next as does the sensitivity of the film. Therefore 200asa is twice as sensitive as 100asa. This is important to remember when understanding exposure.

Camera / Lens / Film / Exposure / Composition