How should I paint the light in shadows?

Blog reader Ewan Lamont says:
"I was interested to read that you 'need to tell myself to paint the shadows darker than they appear, because the tendency (for me at least) is to overstate the fill and reflected light'. John Ruskin noted that the human eye is far more sensitive to light than photographic paper (I am not sure about digital arrays of sensors) and wanted artists to paint what they could see in shadows and which was invisible in the dark shadows of photographs where shaded details did not register. He also deplored the Claude glass for the same reason."

John Sell Cotman, Chirk Aqueduct, 1806-7

Thanks for those interesting thoughts. Ruskin is correct in saying that our eyes can see more light in shadows than cameras can see, especially any cameras that he would have known. 19th century cameras had much less latitude than modern digital cameras are capable of. He's also right that usually we want to avoid indiscriminately copying black shadows in pictures.

But lighting and value organization were never Ruskin's strengths, so I take his opinions about light and shadow with a grain of salt here. I would hesitate to draw any single conclusion about how to treat the light in in the shadows in a picture. There are times you may want to paint shadows black, especially if you want drama. Other times you might want to flood the shadows with variations and bleach the lights. It depends on the nature of the lighting and your goals in a given study or painting.

If my goal in a picture is to capture a sense of light, I'll want to group the shadows and separate them from the lights. I think John Sell Cotman does that beautifully in the painting above. The tones in the shadow are kept together, even though they're not too black. Using a (Claude) Lorrain mirror can help in seeing these tonal relationships. It shows the darks all merged together as a mass -- even though you don't have to paint them as black as you see them.

And look how Cotman unifies the illuminated areas at the base of the aqueduct. It would have been very tempting to put a lot of dark accents and texture into those lights.

Let me leave you with these three suggestions:
1. Paint gives us a very limited scale of values to work with, so management of tone is essential.
2. To achieve a feeling of light, focus on grouping the shadows, simplifying them, and separating them from the lights. 
3. Your reference won't give you the value organization. You have to invent it. It requires a leap of your imagination and a remarkable level of discipline to pull it off.