I posted a Bouguereau (fron artrenewal.org) last night and talked about halftones and their subtlety. Several people in comments wrote that they were unsure just where the halftones were. Let me see if I can explain that. Below is a sphere labeled with the parts of the light. It is taken from a post I wrote in the late 15th century that you can read here.
Most simply understood, the halftones are part of the illuminated side of an object neither in the highlight or in the shadow. Because they are a part of the lights they are part of a family of values (that is degree of lightness or darkness) that are always , always, ALWAYS! lighter than anything in the shadow.
The handling of halftones is one of the niceties of painting where the fine draftsman shows his power. Well understood and handled halftones are a hallmark of a proficient painter. It takes some doing to handle them well, and the naive tendency is to overstate them.
Here is our Bouguereau again, from artrenewal.org. I think I will try to explain this by showing you where the halftones are n0t. I have dropped the head into black and white to simplify this a little bit. Halftones are a part of drawing and thus they remain even when the color is removed.
The little numbered arrows point to some of the shadows on this head. For instance number 6 indicates the mouth, a cavity that the light doesn't illuminate, and 5 points to a shadow cast by the ear. 3 points to the cast shadow alongside and below the nose. All of these being in shadow are DARKER than anything in the lights. The shadows are where the light is not striking the form. That is either the underside of something, a cavity like a nostril, or simply where the form is turned sufficiently away from the light that the light cannot reach it.
REMEMBER, THE LIGHT AND THE SHADOW ARE WHOLLY DIFFERENT WORLDS AND NO VALUE OCCURS IN BOTH!
Above is our girl again. This time the little arrows are pointing to various highlights. These are the brightest areas where the form turns into a position where it, like a mirror, most effectively reflects the light it is receiving out towards the viewer. For instance 4 is the upper part of the chin, 5 is the cartilaginous tip of the nose and so forth. The highlights are the brightest values.
In between the highlights and the shadows is a varied topography created by the close modulation of the values OF THE LIGHT. The swellings and recessions of the form that are illuminated, though less than the highlights, are all halftones and the tell the story of the form as it curves away from the light until it suddenly turns and enters the world of the shadow.
THERE ARE ONLY TWO WORLDS IN THE VALUES, THE WORLD OF THE LIGHTS AND THE WORLD OF THE SHADOWS, AND EVERYTHING IS EITHER IN ONE OR THE OTHER. THERE IS NO OTHER PLACE!
Ok here is the head again, can you find the halftones now?