The way facial recognition works is by looking at landmarks on the face, like distance between eyes, width of the nose, etc. See the link.
You also have iris systems, which look at the pattern of the iris of your eye, and require a good look at the eye. Those, as laid out in the link at my post #27, can scan your eye at a range of 12 meters, currently.
If you have a modern digital camera, you may notice the feature which recognizes faces, so it can set the focus on the face. Take that camera software, and slightly modify it so that when it sees a face, zooms in on the eye.
What would happen if we wore sunglasses?