
Why is the ability to paint accurate value so important? Strong, discrete value zones are essential to create the pattern necessary for strong composition. Early in my painting career I tended to force values into the very dark and the very light, neglecting the subtle and beautiful intermediate values. I got this habit from practicing the art of the woodcut in which I thought in terms of black and white. Others tend to use only the middle values neglecting the deep darks and brilliant lights that give depth.
I have heard the claim that the eye can only differentiate from nine to ten different values. Ansell Adams effectively used a value system where he forced the nearly infinite values of black-and-white photography into only ten values. He developed this theory by observing the more discrete values of landscape painting. John F. Carlson, in his famous book on landscape painting, advocates a similar idea but suggests limiting values to four or five.
By using nine values I can force the values into sets of two or three and still maintain the broad patterns advocated by Carlson, or I can use the entire nine for a more full value, tonalist effect. I find that having nine values on one grayscale gives me more control which is absolutely necessary in the high-wire-act of thick painting.