Value is the relative amount of light and darkness in an image or a particular colour within an image. Value is also known as Tone and refers to the way the artist has represented light and shade.
Value (or tone) is used to create a sense of space and form in images. Learning to show value and tone accurately by depicting folds of drapery was a required skill in traditional artistic studios. The example to the left shows how the illusion of a 3 dimensional object is created by varying the tonal values (from light to dark) in an image.
Using contrast in value (or tone) can also be a very effective tool in drawing attention to a particular part of an image or helping to blend elements of an image together. In the image on the left below there is a small range of tonal values and the elements blend together and no one elements dominates. By increasing the tonal contrast in the image on the right the central star stands out of the image much more and therefore the impression is given that this is much more important than the surrounding elements.
Value is not restricted to Grayscale images but is also a vital part of using colour in an image. By accentuating the tonal difference of the central figure in white to the rest of the image to the right, the eye is immediately drawn to that figure before the others. The rest of theimage is created using colours with similar tonal values and so the visua impact of the rest of the image is very similar.