Colour correction

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Colour correction is the postprocessing of an image so that it more accurately reflects the original scene. The techniques are generally the same as colour enhancement and colour manipulation.

Colour can be defined in a number of ways. By default, Nikon cameras and computers use RGB to describe colour. However, humanly speaking it is often easier to think in terms of Hue, Saturation and Lightness HSL or HSB, and, from the point of view of visual corrections, the LAB colour mode offers powerful visual tools.

Key tools for colour correction include:

• Curves which function differently in LAB from RGB mode
• Levels which set the brightest point of the image, the darkest point, and what should be the midpoint.
• Hue/Saturation which, in RGB mode, allows the user to vary the hue, saturation and lightness for the entire image, or for a colour range.

In many applications, a white point, black point and or neutral grey point can be selected using the colour picker in the above.

These should be used in conjunction with the histogram to avoid pushing the image too far in any direction, and an out of gamut indicator can also be used.

Colour correction is at its simplest when all that is required is an overall change to white balance, as this can be accomplished either when developing from Raw or by setting the white or neutral grey point in one of the tools above. Colour correction can also be used to deal with overexposure or underexposure, within certain limits, and when the image is over or under saturated. A colour cast can be removed either by applying the complementary colour filter, or by setting the white point, or by rotating the hue.

Colour correction can also be used to match the colour in one image to an identical colour in another image.

Colour correction becomes more taxing when multiple light sources of varying colour temperature give a number of different colour casts to different parts of the image. Extensive use of layers and masks may be required to achieve an acceptable result. This is most often the case when a flashgun is used as fill in flash with either tungsten or fluorescent ambient lighting. Ideally, this should be corrected at the time of shooting by placing the appropriate gel on the flash.