[Karin Wells]

When you have a reference photo, it is important to know how to "read" it. Photographs wrongly tend to clump values on the extreme ends of the value scale. Based on this photo, if you were to do an underpainting or a value study, it might look something like this.

But it does not matter if you use an underpainting or if you are painting directly with color, the values need to be defined thus:

First, you must clearly delineate the difference between light and shadow.

The halftone (where light and shadow meet) is very important in defining the form. The light may meet the shadow very quickly as in a hard edge or meet very slowly as on a gently rounded surface. The picture below does not really show a detailed halftone well.

No shadows on the light side of the face should be as dark as any shadow found on the dark side of the face - despite what you see in your reference photograph.

Also, no lights on the dark side of the face should be as light as any light found on the light side of the face - despite what you see in your reference photograph.

Your reflected light should not break up the pattern of the true shadow. Reflected light is a good excuse to add color to a shadow without altering the value in a redical way.