TYPES OF FILM
As mentioned before, nearly all types of film emulsions available can be obtained in the 35mm cassette for miniature cameras. This gives the photographer freedom to work in virtually every lighting condition and moreover to express himself photographically using different mediums and in this also his work is analogous to the painter’s.
We will classify film types in four main categories. These are monochrome negative, monochrome transparency, colour negative and colour transparency. But before we treat each of the categories we will mention the basic underlying factor. All emulsions rely on the sensitivity to light of silver bromide crystals dispersed in a gelatine medium. Furthermore the sensitivity of the particular emulsion is proportional to the size distribution of the silver bromide grains. So, in general, a fast emulsion will have larger grains but also larger size distribution. Slow emulsions will have smaller grains and less size variation in the emulsion. Once exposure takes place and the emulsion is developed chemically, black silver grains will be formed from the exposed grains of silver bromide. Since the emulsion contains grains of different sensitivity, different tonal values are reproduced, but in reverse at this stage. This first stage is common to all film types and is the chemical reduction in the developer to obtain a negative image.
The first films produced were such negative films. Monochrome prints were produced by a second exposure on to an emulsion-coated paper. These first emulsions were more sensitive to blue light, but the later panchromatic emulsions had a sensitivity corrected to approach that of the human eye. The image is always made up of black grains of silver. The size of the grains is larger in fast emulsions. Today a wide range of monochrome emulsions are produced, from 10 ASA, which is slow, fine grain and of very high contrast, through 100 ASA, a medium emulsion, to the 400 ASA, a very sensitive, grainy and wide exposure latitude emulsion. A recent development in monochrome emulsions has been chromogenic film in which the silver grains are replaced by a colour dye during the chemical development stage. In these films the grain size can be kept very small while dramatic sensitivity to light is achieved (about 1600 ASA). A brief mention of monochrome transparency film is worthwhile as a photographer working with this medium can produce brilliant and extremely detailed images but with rather limited emulsion speeds. Remember that since monochrome emulsions are thin they can produce an image definition of excellent quality with good contrast range and freedom from halation.
We now come to the colour emulsions. These are more widely used today by most photographers because they add the dimension of colour to the image produced. Colour emulsions have developed a great deal in recent years due to continued research on the basic tri-pack colour film of thirty years ago. A large range of emulsion speeds is now available with a tendency towards high speeds in both negative and transparency film. The relation between grain size, emulsion speed and exposure latitude still holds as in monochrome, but to a lesser extent, since the silver grains are replaced by colour dyes in the chemical development process.
This article has dealt with the technical aspect of emulsions for the simple reason that it is the basic working material of the photographer. A good knowledge of the characteristics of his working medium will enable him to choose the best type of material for the intended result. Remember that the photographer can use colour distortions and variation of contrast range in an expressive way, and a large number of permutations are possible by manipulating the sensitive material. A photograph is an illusion of reality and its effect depends on the technical and artistic manipulation of the medium. This will be treated more fully in the next two articles.
RL 25/4/85