Here is a picture of Emma Zorn, wife of tjhe Swedish painter Anders Zorn (1860-1920). Zorn, Sorolla and Sargent are the triumvirate of alla prima figure painters from the late 19th century. They have a lot in common. I want to show a Zorn head and note the similarities in the handling of light and shadow that he shares with John Sargent. Zorn was also an admirer of Velazquez.What I want to point out is the "clean" delineation of the shadows in this portrait of his wife.
Zorn has "mined" the shadow areas in this head. There is a complete demarcation everywhere in this head between the halftones and the shadows. Sargent and Zorn both did this, and it gives a concise look of understood form to their art. I have been pointing this out repeatedly bin Sargent's paintings but here is Zorn doing the same thing.
THIS PRECISE DELINEATION IS THE RESULT OF CLARIFIED OBSERVATION, BASED ON A CONCISE UNDERSTANDING OF THE FORM.
I hope I am not beating you over the head with this too much, but this is REALLY important.Great figure and head painters know exactly where the shadow is, and where the halftones (part of the lights) are. They never mush the two together, even in the subtlest passages the delineation is there. This is installed from an understanding of the form and may or may not be actually visible. But here is a little trick you can use to find this demarcation yourself.
YOU CANNOT CAST A SHADOW INTO A SHADOW!
That means,that if you are confronted with a passage where it is unclear where the halftone ends and the shadow begins, you can hold a pencil or stick of charcoal or a brush between the light source and your subject, be it a head or still life, the shadow of that stick will appear in the halftones and not in the shadows. Try this, it is an amazing phenomenon. The shadows will be unaffected by a shadow thrown across the highlights .
Every time your brush touches that canvas you must know for sure whether what you are painting is in the shadow or in the lights. Every time. There is no other place. If you have a doubt, cast a shadow across the model and see what happens. If I have not explained this clearly, let me know in the comments and I will take another run at it.