Separation of Light and Shadowedit
In the image above, I’ve taken Sargent’s portrait of Henry James and applied a threshold adjustment layer in photoshop at a value of 70. That’s 70 out of 255 (using the RGB color scale) where 0 is pure black and 255 is pure white. The gray color around the image is this same grayscale value. It’s approximately on the edge of the light and shadow on James’s face, and shows the separation of the side of his face that is in the light and the side that is in the shadow. This separation is one of the big principles of creating the illusion of solid forms in a painting that you want to read as realistic.
Often when we’re first learning about color and values, we tend to overdo the tonal variations within one of the sides, which leads to a mottled effect, breaking up the composition and breaking up the sense of form on the image. We’re much better off erring on the side of keeping everything closer together as Sargent shows us. It both reads more clearly, realistically, and creates a stronger composition because the shapes become unified.