Posterizing
 

Here's a way to sort your photographic, or other source reference material, into three or four simple values. Use the IMAGE -> Adjustments --> posterize menu. Using this function will translate your photo into a few clumped values. Not too good for detail work, but it will give you an idea of the value pattern(s) in your source material.

You can tweak it a bit to increase or decrease the values and areas of differing values.

Finally, if I'm covering old ground, I apologize.