A few words on keying a painting, as related to this wonderful Tissot:
When painting, you can't copy directly the value range in nature: the gap between the lightest light and the darkest dark. Your paints aren't bright enough or dark enough to do that. (1/)
When painting, you can't copy directly the value range in nature: the gap between the lightest light and the darkest dark. Your paints aren't bright enough or dark enough to do that. (1/)
Keying a painting is selecting a scheme for arranging your lights and darks such that it allows you to convey the light-effect you want. Consider this Emil Carlsen: he's pushed his darks dark, giving him space to make his midtones dark, leaving space for the highlight to pop (2/ 
Compare this Emil Carlsen. Objectively, the highlights are brighter than in the previous, but they don't pop in the same way: he's keyed this one such that most of his range of values is consumed by modulating halftones, so there's no space for the highlight to stand out. (3/)
Going back to the Tissot! He's trying to do several things here: convey a sense of outdoor sun and shadow AND not crush anything so far it vanishes AND do it almost purely with value, not with color contrasts like Monet would (4/
He succeeds by carefully grouping his values, and keeping things flat and with minimal variation within value-groups. Most of the shadow-zone is almost the same, giving space down for the black dress to be black, and up for faces to show form in the reflected light. (5/)
Similarly, that light patch is kept within a very narrow range, giving anything in light as much space from the stuff in shadow as he can manage. Again, it's got to be, because he wants the sense of brilliant light more than he wants modeling of form or delicious highlights (6/)
But yeah. Keying a painting, in essence, is stretching or compressing lights and darks to convey the impression of the particular aspect of light you wish to convey. Tissot, imo, kicks ass here at a difficult task.
(7/7)
(7/7)
One more: I'd suggest that, if such a thing exists the "standard" key is kinda like this. Shadows kept low & flat, but with space for darker accents (pupils, nostrils). Lots of space in the midtones for modelling, kept fairly light but with headroom for a subtle highlight (8/7)
