Here's my self portrait for the Color and Light assignment. I've done value studies from photos before, but I rarely get the chance to do something from a mirror like this. It was interesting to see how the eye sees values differently from a camera.

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I learned that thinking in mass light and mass shadow early on really helped smooth out the process.
Transitions/Half Tones can get pretty dark, but there's usually a very distinct jump in values between it and the mass shadow.
Highlights appear on the planes that direct the most light towards the viewer. Depending on the surface, the value jump a highlight has varies, such as the forehead being more matte than the nose.
The mass shadow has a lot of subtle value changes that are all important for defining form.
The reflected light can be very subtle, but having it there gives the mass shadow more form.
The core shadows seem to mostly appear in the folds of things, where it's difficult for any light to get inside.
Cast shadows seem to have a sharper value jump than the border between the half tone and the mass shadow. Probably because of the lack of transition from the half tone.