One of my goals is to learn how to draw faces and portraits. It's a challenge to get proportions correct and placement of facial features. Ever since I took a two day workshop from Anne Bagby in 2004, I've collected photos of faces and practice drawing them occasionally.
When we sketch at coffee houses in the winter, I bring my little box of faces and practice drawing them. To see how my drawing compares to the original photo, I scan each into Photo Shop and over lay them to scale. The drawing is on the bottom and the photo on top with an opacity of 50%.
Now I can observe both layers to see how they relate to one another.
This tells me I have to work on the placement and spacing of the eyes and mouths. With practice, these drawing will become more accurate.