Came across this studio lighting guide – this should be real handy for painting portraits or rendering too.
When creating 3D scenes this can help a lot also. But even for painting it’s a good practice to imagine all light sources before starting. Not even just the main light. In reality there are always several lights around, i.e. when the main source is reflected back. Some you barely notice, but keeping them in mind helps for things like reflections (e.g. for portraits in eyes or glasses) where the lights show up much stronger. This guides the eye to see the shapes clearly.
So it’s useful for realistic rendering of the shapes, but the placement of the light also affects the mood of the picture – and the expression of the subject. Just think of the “ominous light from below” trick, or having half of the face in a black shadow.
Another great helper is this video with a rotating light source – if you have want reference for specific light directions (usually hard to find via google).
Here’s a gif part of it. (Also I downloaded the light rotation video in case the video link should ever go down).
And I have another little guide that explains how to apply it when painting. It helped me a lot back when I found the image, so I still have it in my reference collection, but don’t know where I have it from.
In short it shows: Simplify the face into planes. Then modify the local color to change according to the angle.
One should of course keep other local color changes in mind – in the face the cheeks are often more red, and the jaw (more for men) is more grayish. But now that I think of it, it’s enough to make another page later with some more detailed guides.