A lot of aspiring painters have a hard time with accurate rendering, whether painting from life or just out of their head. So my first few posts will address some key properties of light and surface.
Some people seem to get this stuff intuitively. I'm not one of those people, so I often have to figure things out. One thing I've found that helps is working like a 3-d rendering program: thinking 3-dimensionally, rendering effects in passes, and in general isolating problems to deal with them separately.
So let's start with Lambertian Reflection, which is when a surface reflects light in a way that each part of the surface looks the same from every angle.