Colour cast

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A colour cast is a tint of a particular colour, usually unwanted, which affects the whole of a photographic image evenly.

Certain types of light can cause film and digital cameras to have a colour cast. In general, the human eye does not notice the unnatural colour, because our eyes and brains adjust and compensate for different types of light in ways that cameras cannot.

In film, colour casts can also be caused by problems in development. Improper timing or imbalanced chemical mixtures can cause unwanted casts.

[edit] Solutions

Example of a photo with an orange colour cast due to incandescent light

The same photo with the colour cast corrected

There are two main causes of colour cast, Sunlight (or cool light) and Incandescent light (or warm light). High end digital cameras can automatically detect and compensate colour cast. Otherwise, photo editing programs, such as Photoshop, often have built in colour correction functions. For film, blue filters and amber filters are used to counter the cast. Amber filters are used to ward against the blueish tint caused by daylight. Blue filters fight the orange colour caused by incandescent light.

Many camera companies supply a wide variety of coloured filters in varying degrees of intensity. Kodak's amber filters, for example, vary from palest yellow ("81C") to deepest amber ("85B"). A photographer choses which filter to used based on the condition of the ambient light. Exposure meters can read the temperature of the existing lighting conditions and guide the selection of the filter. Clouded sky, for example, requires a paler amber than clear blue sky. If a filter is unavailable, flash is an alternative solution which usually provides enough neutral white light to counter the cast.

In the case of film, if photographs all contain the same cast, it is usually indicative of improper chemical development. If the film itself does not contain any cast, it can be reused to create another set of photographs in proper chemical conditions. If the film contains a cast, filters can be used during photo development to correct it.