>>384777842
Here's the same set of objects with no matcaps, but basic shading instead. Notice how they were actually being lit from the right, but the first matcap in the previous example made it look like it was being lit from the left. Since matcaps are stretched over the model from the camera's view position, they're completely static and will show the material being lit the same way from every direction.
Also notice that the face is no longer visible, since it was just a normal map that reflected the matcap in different directions.