Above is the head of Daphne from a larger statue by Gian Lorenzo Bernini 1598-1680.
Heres an example of light falling across a figure and it presents something like the usual drawing problems. The halftone of the left cheek shows nicely the separate world of halftone approaching the shadow. However the halftone and the shadow in the eye sockets are very close together, the knowledge of anatomy is helpful in knowing what is halftone and what is reflected light.
A little trick that often works in the studio when painting a head or a still life, is to take a brush handle or your charcoal and cast a shadow on the subject. You will find that you cannot cast a shadow onto a shadow. If you can cast a shadow onto it, its a halftone.
In practice it is often very hard to determine where the halftone leaves off and whether the reflected light is as bright as the darkest part of the lights, particularly on a dark or extremely reflective surface. So, out in the world our nice system of light and dark is not so easy to see and often you will find situations where because of multiple light sources or materials of varying reflectivity the object doesn't seem to display the perfect division of the two. You may see situations where a very dark object in the light, or a very dark object in the shadow seems to escape our rule also. It happens. Although in the physics lab it is probably an immutable law out in the real world things can get awfully fuzzy. So I guess the systematic representation of light is part science and part artistic convention.
For expressing form this practice works and ignoring it does not. But it is a system like perspective, a general principle that helps the artist in his task. It is a model for the real world and not the real world itself. You can find scenes that seem to defy perspective but you may not make a convincing representation if you draw them that way.
OBSERVATION SHOULD NOT RULE , BUT SUGGEST EXECUTION
Drawings are routinely ruined for lack of understanding of of values and seldom harmed by it. There are plenty of examples of figures with lots of dark halftones and strong reflected lights. Michelangelo did many, but he was an artist who understood this principle well enough to cheat it.
The nice thing about knowledge is that if you acquire it you can choose to operate without it. The problem of not having knowledge is you can't choose to operate with it. There is an intellectual laziness I have on occasion encountered in the art world, that derides the study of academic technique. Often these folks fear their natural abilities will be diminished by such studies. They imagine that ability is bestowed rather than achieved. ( I am not speaking about you, anyone you know, or have ever met ). Go for the full tool box it is not any heavier than the half empty one.
Good painting is the result of both observation and knowledge. It is an old artistic saying that:
first you paint what you know,
then you paint what you see,
and finally, you see what you know.
What that means, is that beginning painters don't really look at what they are painting, instead they make a pictograph or a symbol containing the things they know about their subject. Think of those drawings grade school kids make when asked to draw their own house, that contain a door ( gotta have one of those ) a window and a chimney but are a representation of every house, the idea of a house, rather than their own particular house.
As artists progress they learn to see things in the abstract, that is they can see the shape of an object laid out in space, what the thing looks like, which is often very different that what the thing looks like in the head on or side view we store in our memory. This leads to more accurate representation including proportion and values..
In the final stage the artists' eye is informed by ideas like conservation of values, anatomy, color theory, perspective etc., and when he peers put at the world he actually sees those principles operating out there.
A woman in a class once told J.M. Whistler ,"but I like to paint what I see" to which he replied "wait till you see what you paint".
I guess if I am going to quote Whistler I should show you one. I will review his book "The Gentle Art of Making Enemies" in a later post.