I came across Petteri Sulonen's article - Mastering Wide Angle. That woke me up a bit.
On focussing, he says:
- If your camera is even slightly out of the horizontal, verticals will converge
- "Short focal lengths have lots and lots of depth of field." They sure do. However, the other side of the coin is that the attendant wide scenes tend to be very deep, so you're going to need all of it, and more, if you want to keep everything sharp - The trouble is that while there's lots of DOF, the scenes are very deep, and wide-angle scenes tend to draw the eye to "wander" around the frame, which means it'll eventually land on something that's not critically sharp. A second complication is that auto-focus really doesn't work that well with wide-angle. Especially with the shorter, darker lenses hyperfocal distance wide-open is really quite close. Moreover, objects even a few meters away may be quite small in the frame, while the AF sensors are quite large. This means that AF will have real trouble latching onto anything more than a few meters away.
- you as the photographer have to take flare into consideration anyway; either work it into the composition or try to minimize it while composing.
- Expose for maximal dynamic range, then correct in post-processing.
- Stop down as far as you can.Identify the visual center, and focus on that.
- If you want infinity to be sharp, focus on infinity.
- If you have to choose, focus behind rather than in front of the subject.