This photo works partly because I managed to focus on the bridge of the sunglasses and to keep it there even after I stopped down a bit. As this post explains, accurate focus is sometimes easier said than done.
Caution: Don't read this if you're already happy with the results you get with your DSLR or if you never use lenses with an aperture faster than f/4. If you do read further, then don't blame me if become obsessed with finding ways to wring the last iota of focusing accuracy out of your camera. You have been warned.