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As is obvious from the name, contrast filters are used to enhance colour contrasts in photographic images. Contrast in images can also be increased digitally through various software, but a contrast filter while shooting has a different effect altogether. Contrast filters basically work on the opposite colour transmission theory, according to which in the visible spectrum, a particular colour absorbs the light of the same colour and then transmits the opposite spectrum wavelength through it. So if a green colour absorbs the green colour and transmits red colour in a particular frame, then a red filter would absorb red colour and transmit green color

There are different kinds of contrast enhancement filters used in different conditions and for different films. For black and white films, yellow coloured filters are used to increase the contrast between the sky and the clouds. Yellow filters darken the sky, thus bringing out a heavy contrast in a frame with large parts occupied by sky. On the other hand, coloured films use all colours of filters as they absorb the same colour and transmit the opposite colour. With a colour filter, like the Cokin A 007, the tone of one colour decreases, while the tone of opposite colour decreases. For instance, a deep green filter will let the green plants and trees in the frame remain at the same tonal level, but will darken the colour of the sky, thus bringing out the contrast. Contrast filters are generally used in over exposed frames or frames in which light is perfect, in order to retain the details. However the contrast filters also wash out the colours, thereby allowing you to retain the exposure and colours, without having to close the aperture.