To determine where values change when light moves over an uneven surface by using a range of values to paint the different values in a realistic, monochromatic self-portrait;
To effectively communicate an image that looks like you through careful examination of the values and structure of your face
I think that a necessary skill to paint facial features is being able to accuratly differentiate two similar but different shades and be able to recreate it with paint. Being able to use an accurate shade of grey/black for the iris is a necessary skill so that you can correctly portray the eye with good detail, and even better when applied to the area around the eye. Using slight color changes around the eyes and nose is good so that you can accurately depict the shading of the area.