By Tarak Ch. Sarma, Senior Faculty, PVTI

Whoever has taken a photograph with a camera, has been photographed or even seen somebody take pictures, knows that one has to load the camera with a film i.e. one has to put a film in the camera.

What is this film and what does it do? It is a transparent sheet of some substance called acetate. On this acetate strip a coating is given of a suspension of silver halide in gelatin. Silver halide is a crystalline substance like our common salt. The suspension is called the emulsion and the acetate sheet is called the base of the film. The emulsion on the base is the most important thing photographically to us in the sense that when light falls on it, the silver halide of the emulsion undergoes some change, while the areas of the emulsion not striken by light do not undergo any change. We say that silver halide is light sensitive (we also say that the film is light sensitive) and the more the light, the more the change. But whether the change in the silver halide is more or less, the change itself is the image of the thing where from the light has come. But this image is so faint that we can not see it unless it is improved (we say developed) a thousand million times. The invisible image we call the latent image. So we can say that the film is the heart of our photographic system.

What we have said in the mean time is that we have to allow light from some thing to fall on the film. In photographic jargon we say we expose the film (to light) and we get on the film the image of the thing from which the light has come.

Now to get a good photograph we have to know some very important things about the film, like:

1 . The film i.e. its emulsion is light sensitive. But all the different films have different degrees of sensitivities. The more the sensitivity, the less is the amount of light necessary to produce the necessary degree of change of the silver halides of the film. Manufacturers of the films indicate degree of sensitivity of each film by a number called ISO (International Standards Organization)—larger ISO number means that the film is more sensitive to the light and as such less light will be necessary to produce the required amount of change in silver halide.

1. The ISO number may be any of two series of numbers to indicate the same sensitivity. A film may be of ISO 100 or it may be of ISO 21 0 , but it has the same sensitivity—the manufacturer will mark both these two numbers on the film packet as

ISO 50 / 18 0

ISO 100 / 21 0

ISO 200 / 24 0

ISO 400 / 27 0

1. As ISO 100/21 0 film is twice as much sensitive as an ISO 50 / 18 0 film, the same way an ISO 200 / 24 0 film is twice as sensitive as ISO 100/21 0 , and so doubling the first number of the twin is to double the sensitivity of the film, while the second number in the twin increases only by 3. So if you buy an ISO 800/30 0 film, you have bought a film twice as sensitive as a film of ISO 400/27 0 film.

Doubling the sensitivity of film means that it will need only half as much light than formerly. For example a film of ISO 200/24 0 needs only half as much light that is necessary for a film of ISO 100/21 0 .

4 . films are classified as Black and White (B&W) negative, B&W positive, Colour negative or Colour positive films. The positive film is also known as slide or Transparency film.

A B&W negative film produces first a black and white picture on the film, in which white in the scene or object becomes black and black in the object becomes transparent, and all other colours in the object between white and black come out as mixtures of black and white, which means grays. In this form the film is called negative. So a B&W negative film first produces a negative image and this negative has to be photographed to get its negative again. The second negative is obtained usually on a paper base. The second negative being a negative of a negative, is a positive, meaning its whites are whites of the original scene or object, similarly for the blacks and grays. This process of getting negative of the first negative is called printing.

So when we photograph on a negative film, the image obtained is a negative and we have to print it to get our desired photograph.

The steps to be followed in case of a colour negative film is similar. Here in the first negative we get yellow, magenta and cyan in place of the subject’s (i.e. the scene or object photographed) blue, green and red areas respectively. In colour photography blue, green and red are called primary colour, because all the colours around us can be obtained by mixing these three colours in different proportions. So the first colour negative is in yellows (opposite of blue and is a mixture of green and red), magentas (opposite of green and is a mixture of red and blue) and cyans (opposite of red and is a mixture of blue and green) and on printing this negative we get the colour photograph of our original subject.
film_2 film_3
Daylight film under tungsten light

Tungsten light film under daylight quality tube light
or in daylight

The process followed in case of B&W or colour negative films is a negative-positive process, since we first get a negative image and from that get a positive image later. But there is another process in which the positive image or photograph is obtain directly, i.e. without first obtaining a negative. This process is known as direct positive process. The film is usually called a reversal film, whether for B&W or colour.

1. Another important thing of note relates to colour films. The colour films are of two types, daylight and tungsten light. The daylight type film is for taking photographs during daytime, usually two or three hours after sun rise and before sun set. This film can also be used in daylight type fluorescent tube light. The daylight quality light is a bit rich in blue content. The other type of colour film is for taking photograph under tungsten filament bulbs. This light is a bit rich in red content.

If we take photographs during daytime on daylight type colour film, the colours come out well; but if the film is of tungsten light type, then the photograph will have bluish tinge or cast all over. In the same way if we photograph under tungsten light on tungsten light type film, the colours come out well; but if this film is used during daytime or under daylight quality fluorescent tubes, the photograph will have a bluish cast all over. Also if a daylight type film is used two or three hours before or after midday, the photographs comes out a bit reddish; but many people call this cast golden and like it.

This is for today. In our lesson 4, we will be talking about exposure.