I want to discuss the specific experience of painting from photographs in a way that develops skill rather than producing technically competent but inert copies.

The difference: a skilled photo study asks 'what is the light doing here and why?' A mechanical copy asks 'what color is this area?'

The practice: before making any mark, spend five minutes analyzing the reference. Identify the light source direction and color. Identify the shadow fill color. Identify the three most important value relationships. Only then start painting.

Midway check: stop at the halfway point and compare your painting to the reference specifically for value accuracy. Values are the most important element to get right. Color errors read as style; value errors read as mistakes.

The finish question: when is a photo study done? I stop when I've learned what I set out to learn, not when the copy is complete. Sometimes the useful learning is in the first twenty minutes. Finishing the copy adds execution time without additional learning.