I wanted to cover lighting and form a little more in depth.
" The form principle is the analysis of nature in terms of geometrical solids which can be rendered according to laws of tonal contrast."- James Gurney Color and Light
Put simply everything we see, we see because light is hitting an object and reflecting back to our eyes. We see form in aspect of light and shadow and color.
That said, we understand our world through value.( color is important but we'll get there later.)
Every object be it a simple sphere, a head, a cube, every object we see and paint will be affected by light the same way( except translucent and transparent materials)
In the following example of a sphere, it is an object with a single strong directional lightsource, as we might see with the sun at certain times of day.
What you will notice on this sphere is a strong division between the "light side" and the "dark side"
On the light side, you have the Highlight, which is the point at which the direct light is hitting the object and "blowing out" all other value or color. On most materials this appears to be white or near white. note that it is not on the edge of the sphere but appears inside the form of the sphere showing us that the highlighted part is closer to us than the flat visual "edge" of the object.
As we move our eyes over the from away from the highlight the actual color of the object begins to appear. That is the Center Light which is the closest to the most unadulterated value or color of the sphere. In other words, its Local color or value.
The Halftone is where the form starts to move further away from the highlight and is getting darker but the light here is still stronger than the dark areas.
Next we hit the Terminator Line. The terminator( aka the shadow edge ) is the division between light and dark. It is the darkest area on this sphere and the darkest area involving the sphere after the occlusion shadow. This is where the "dark side" begins, after this, we begin to see more lighter dark values due to light reflecting from the rest of the world. The terminator line appears as a 90 degree line( which curves over the surface of the object) to the light source as you can see in the second example( the axis line.) At the terminator all surface texture seems to disappear. Any textures on the surface, like skin texture, is difficult to see in any of the shadowed areas, but at the terminator line disappears completely possibly to return in the reflected light.
The core shadow is right behind the terminator line in darkness and only exists if the secondary or reflected light sources don't overlap with the main light. The core shadow is important to maintaining the illusion of form in your drawing even if you don't see it on your model/reference.
Reflected light every object exists in space and since every object is reflecting light, all these objects will reflect on each other. Since the direct light is strong, the reflections can only be seen in the shadow. So as we move further away from the strong light, the wold reflects back into the shadow creating a lighter area on the object.
The Occlusion Shadow is the point at which the sphere or object meets another object. No light can get in between the two and so therefore it is the darkest area in this image. It is quite literally an area without light...complete and total darkness.
The cast shadow is the shadow the object casts upon the surface it is upon or against.
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Can you see these light principles at work on the Lincoln bust? The light source is a 3/4 left light source like the sphere above.
The cubes below show how the same concepts apply. Note the grey cube and the white cube are more likely than the black cube to have reflected light areas and the terminator line defines the hard edge of the cube rather than the soft edge of the sphere or lincolns head. Look for ares of Hard Edge and soft edge on the Lincoln portrait.
This is an example of diffuse light on a sphere. Its the kind of light you might see on an overcast day. Note that there is no distinct light side, shadow side terminator or core. all of the upward facing planes are lighter because the light is coming from many angles rather than a direct light.
Its important as you work on your pieces to recognize where the light source is, and how you can best express that in your image.
Again with the lincoln portrait, look at places where like the cube, the planes shift direction like the temple, the cheekbone, the nose, the chin and even less dramatic ares like the lips and eyelids. There will be a harder defined edge here( like the cube) where as on the forhead, where the planes are only slightly different at angles( more like the sphere, you can see the highlight and a gradual shift to the center light and halftone. Approach your portraits with this in mind as you move forward.
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