Often, when teaching a class of students using photos as reference material, I find myself pointing out things that they haven't seen in the photo, or correcting their drawings with reference to the photo. But the duplication of a reference photo in its exactitude is not precisely "ART" (although it is handy as a tool for learning the basics).
Sometimes I will tell the student to stop(!) painting on a painting in progress, not necessarily because it is "finished" but because it communicates. (Often they don't listen to me, because (after all) it's their painting, not mine.)
it is painted with the palette knife, and it could be more "finished", but this painting communicates a mood. The small bits of light orange coming through the clouds are emphasized by the surrounding dark. The photo below is the one students are working from.