3 Check the portrait at twilight.
People always think that the lighting in a painters studio should be abundant. However in some cases, the opposite is true. A few times scarce lighting is very welcome. When I want to check an almost finished portrait I switch off the lights in my studio to see the portrait at semidarkness. Then I notice other differences in hues and values than I’d seen in full lighting.
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See also this article on studio lighting.