What makes a camera/lens focus quickly? I've noticed that some lenses focus faster then others and some cameras focus more quickly then others. It also seems that a lot of the f/2.8 lens focus more quickly then slower lenses. Is this just because they are generally more expensive lenses and built better or a direct correlation to wider aperture? Which is more likely to focus quicker, my D50 with a fantastic lens or a D200 with a cheap kit lens? Are there more variables in play here?