Many guys may be confused in art. What happens?
Actually if we talk of light and shades, there is only one point. The light and shades follow law of cosine and is not directly proportional. A point under 90 degrees is fully bright. While at 50 degrees it is dark. One need not be at 0 degrees to be in shade. A 50 degree is shade enough. Because the light and shade follow law of cosine.
Now confusion happens because
1. There may be 2 or more sources of light.
2. A surface may be oily or mirror like causing reflection.
3. Man wants to earn and is greedy. So he makes methods for intricate details. Like using Andrew Loomis head method etc. to get depth in drawing. Also more knowledge pieces of a subject or art media gives the art an edge.
4. Certain presentations like perspective in art makes things attractive. But then perspective is difficult.
5. While nature is symmetric our writing or art making is not. You involve only one hand and that too two or three fingers including thumb to do art work.
I believe I have an edge in point No.5. Naturally certain points arise inside me which makes me see the points raised in 5 easily.