I don't know if this is worth sharing, but I found the exercise helpful. I think Kim English is a master of light... his paintings just glow and I've been trying to figure out how he does it. So I copied one of his painting images from a gallery website, and turned it to grayscale in Photoshop. I wanted to see how his values break down. He seems to have the full range of values from 10 - (black) to a 1 - pure white.
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An important part of a successful painting is of course, having a one range dominate the scene with accents of the other. And I think that connecting those value ranges is important so that your eye doesn't jump around the painting too much.
My scene has a few less values. And midtones seem to dominate as usual.
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Honestly, working from photos makes me worry that I would get too dark or too much a range of values. When I work
en plein air, the values are closer generally. At least I always get them close. With so much light bouncing around you don't see very dark shadows. My trouble with painting on location (landscapes) is that I seem to end up with something looking like it was all painted with 5 values... midtones. I've always had to really push myself outdoors to get a range of values. Maybe I need to push more in the studio too!