Have the students compare the developed films produced by colour transparency and colour negative films. The students could map the colour opposites by comparing images.

Some film can be used to produce either negatives or positives. It is the development process that determines the end product.

The most popular film is colour print film that produces a negative that is then printed on light sensitive paper, similar to black and white photography.

Colour transparency film produces a slide intended for projection that is a direct positive image. A print can be made from a transparency by using a special printing paper, different chemicals and processing procedure.

Draw a scaled cross-section of colour film to show the relationship of the film's layers.

Colour film is composed of three emulsion layers, sensitive to red, green and blue light that produces an image that appears as the inverse or negative colour for the three emulsion layers. The red emulsion layer produces a cyan dye, the green emulsion layer produces a magenta dye and the blue emulsion layer produces a yellow dye. These complementary colours are not the same as for paints. When the print is made, light from the enlarger passes through the film producing the opposite light colour on the printing paper surface. The colour enlarger is adjusted to increase or decrease the amount of cyan, magenta and yellow light that is directed through the negative to correct the colour. A densitometer is used to measure light intensity and provide adjustment for the colour balance for a print. Skin colour is generally used as a guide to adjust colour balance.

The Internet may provide a good opportunity to mix the primary light colours and see the effect of different light mixtures. Refer to the list of websites in the bibliography.

Every film manufacturer has its own formula for the various layers that create the three colour layers. Some films record red more intensely than other films. Some record greens more intensely. Some photographers use different brands for different types of subjects, preferring one film type for portraits and another type for landscapes.

Students could experiment by taking photographs of the same scene using different brands of film and comparing the photographs.

Colour film also reacts to light in a fashion similar to black and white film. There are a variety of ISO speeds available for colour film and, as is the case with black and white film; the larger the ISO number the more light sensitive the film is. As well the more intense the light, the greater will be the chemical reaction within the crystals of the three colour layers.

Colour temperature is measured in degrees Kelvin (°K). The colour of light emitted by a source is compared to a heated body at different temperatures (called a black body).

An example to help to understand this concept is to have the students observe an iron bar as it is heated. A dull yellow-red light colour is the first to appear. As the iron is heated, the temperature increases, the light emitted becomes progressively more blue: the higher the temperature, the bluer the light. Therefore the reddish light is said to be cooler than the blue light, and has a lower Kelvin temperature.

Different photographic situations will require a film balanced for that colour temperature range. Films rated at 5000°K and lower are referred to as Tungsten film and produce a natural appearance in the subject at lower colour temperatures.

Those colour temperatures that are higher require a film balanced to higher colour temperatures, referred to as Daylight film.

Colour film is available in Daylight and Tungsten types. Daylight films are colour balanced for the average light conditions at noon (5500 °K). Tungsten film is colour balanced for use with photoflood lights (3200 °K).

An 80A filter with Daylight film allows it to be used with tungsten light sources. Using an 85B filter with Tungsten film provides normal colour balance when exposed in daylight.

Have students expose film at different times during the day (e.g., in early evening, mid-day and morning light). The use of slide film is recommended for this work because, if negative film is used, the colour printing lab will colour correct the light effects that are evident at dawn or sunset, and the print may not exhibit the characteristics that the photographer was trying to record.

Compare photographs taken in natural light to ones taken in tungsten and florescent lighting. Have the students shoot in these light conditions using Daylight film. Discuss why colours are recorded differently.

It is important that the students understand that good colour balance results in lifelike flesh tones and colours for clothing that are reasonably accurate. Colours closely associated on the colour spectrum tend to blend easily whereas those widely separated create contrast in photographs.

There are many films available for different situations. Examples may include films for portraiture, landscape and scenery, natural outdoor lighting and artificial (studio) lighting.

The amount of available light will also determine the film selected. The lower the light level, the higher the film ISO number requirement will be for that situation. The higher the ISO number, the less sharp or grainy the image will appear, particularly if the image is enlarged beyond 8x10, for example.