Contrast describes tones, specifically the relationship between the darkest and brightest parts of an image. If the difference between darkest and lightest portions of an image is vast - for example, if the shadows are very dark and the highlights are very bright - an image is said to have high contrast. On the other hand, if an image’s tonal range is more toward grayer tones and the shadows are not very dark while highlights are not very bright, an image has low contrast. A lack of contrast can give an image a “flat” look.