What is value?
Value refers to the lightness or darkness of a color. It indicates the quantity of light reflected.
A basic value scale is easiest to understand, however, in grayscale (without the addition of color):
While value affects the way color appears, it is technically independent of color in concept. Images that have a full tonal range are considered to have “good contrast” because they have a representation of each value in the full value scale. That means the 11 steps above would be well-distributed in an image. This can even be true of images that have a limited palette, as in “posterized” images. In the case of a grayscale composition, the values would have pure white (0% pigment), pure black (100% pigment), and a representational distribution of the values in between (“gammas” or “midtones”).
In a “low contrast” image, the values are limited primarily to the gammas, or midtones. That means there is typically no representation in the image of pure black or pure white.
It is important to note that although we have only discussed value here in terms of grayscale, adding a color to the scale does not change the value range! A useful way to think about how value translates between grayscale and color equivalently is when you take a color image and make a black-and-white copy of it on a copy machine. The color is eliminated, but the values are still identical to the original color image. Look at the examples below for a better understanding: