Non neutral color balance - cast

Loss of neutrality, in this context, can be defined as the state when any

one colour is influenced adversely by another. It is easier to identify a

colour shift from a neutral than it is to gauge the influence of one colour

within another strong colour. In most instances the degree of shift will be

slight and, therefore, have to be identified in areas where it will be most

noticeable. These will be in shades close to grey.

Normal colour vision is very accurate at gauging neutrals provided that

the reference information of the surrounding conditions is correct. An

unwanted, visible colour shift is known as a ‘cast’. There is no guarantee

that a colour cast is going to be equal over the whole range of the image.

Incorrect exposure, poor processing, poor film storage and some product

characteristics can lead to tonal colour shifts. Shadows can be moved

from the neutral when perhaps the midtones and highlights are correct.

Similarly, the highlights might need special attention whilst other sections

of the range do not. These special requirements must be identified at the

initial viewing stage. In order to decide how best to cope with a cast it is

vital to identify what type of cast it is.

There are three types of colour cast:

❑ Overall colour cast. In this instance the whole image area is

affected equally by the colour defect. It is usually caused by mismatching

the film used to the prevailing lighting conditions.

❑ Tonal colour cast. In this instance the colour shift is confined to a particular

range of tones: shadow, midtone, or highlight. There may be

more than one cast of this type. There may be a different colour

affecting the highlights than the shadows. To fit this description all

shadows or highlights would be affected throughout the entire image.

❑ Localised colour cast. Here the unwanted colour influence is confined

to a particular geographical area of the image, not a range of tones. The

cast may cover the full tone range but not the whole picture. Common

causes of this type of defect are unwanted reflection from a strongly

coloured object, mixed lighting (daylight plus fluorescent plus incandescent

– very common in interiors or exhibition photography) or

stray light from a source which may be outside the picture area.

Overall and tonal casts can be removed by global controls during

scanning. However, the only way to remove a localised colour cast is by

retouching - either retouching the original before scanning or retouching

the separations afterwards.

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