Extreme values
If you have difficulty identifying the darkest and lightest values in an image, try "posterizing" a black and white conversion. In this example I've posterized the image at four levels, so I see black, white, and two grays. The black shows me where the darkest darks are located in the reference photo and the white indicates the lightest lights. Just remember that YOU make decisions and can change values as you paint if you like. Don't be handcuffed to the reference photo!
Extreme values
If you have difficulty identifying the darkest and lightest values in an image, try "posterizing" a black and white conversion. In this example I've posterized the image at four levels, so I see black, white, and two grays. The black shows me where the darkest darks are located in the reference photo and the white indicates the lightest lights. Just remember that YOU make decisions and can change values as you paint if you like. Don't be handcuffed to the reference photo!