This is something I don’t see talked about much, if at all actually, but it can save you untold hours of frustration if you get it right.
A basic problem of painting realistically is that we can’t often replicate the full range of values we see in our subject in our paintings. Truth be told, we rarely can.
Because we’re reducing three dimensions to two dimensions, and a flat surface has a more narrow value range than multiple surfaces, some of which will be facing towards the light, and some away from it.
Hopefully this video will explain it!
Best wishes and thanks for watching,