There are several important ways in which colours interact. Because of the psychology of perception, they are not always as most people would expect.

Here is a set of phenomena that has to do with the way we process colour in the visual cortex, and they have an important bearing on theories of harmony and discord – and so on the ways in which you can present colours in a photograph.

The two most important of these are successive and simultaneous contrast, first identified by the French chemist Michel Eugène Chevreul in the 1820s. Successive contrast, also known as after-image, causes the eye to ‘see’ the opposite hue immediately after looking at a strong colour.

Stare at the red circles (below) for about a minute, focusing on the small cross in the middle. Then quickly shift your gaze to the cross in the centre of the blank white square. You should be able to ‘see’ a circle of a different colour – blue-green (the same effect occurs if, after looking at the red, you close your eyes tightly).

This after-image is a reaction generated by the eye and brain, the effect is strongest with a bright colour and when you have stared at it for a long time. The significance of successive contrast is that the after-image colours are always complementary, meaning directly opposite on the spectral colour circle.

The companion effect to successive contrast is simultaneous contrast, in which one colour, juxtaposed with another, appears slightly tinged with the complementary of the second. This is at its most obvious when a neutral appears alongside a strong, pure hue, as seen in the illustrations and the photographs.

In other words, a patch of grey on a background of a single hue appears tinged with the opposite colour. Although much slighter, this same contrast occurs between two noncomplementary colours; each seems tinged just a little with the opposite.

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<h2>Circles effect</h2>
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A variation of this effect includes the white ground. Stare at the cross in the left box. After at least half a minute shift to the right cross. You should see a cyan circle.
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