Focusing technique

The second problematic area in wide-angle technique is focusing. 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. So, it pays to be extremely careful about deciding where to focus: which part of the scene is the spot where the eye is drawn to and "rests." If it's near infinity, then focus on infinity and let the foreground go slightly soft. If it's in the foreground, then make that sharp and let infinity go. Again, because the scene is complex and often based on a figure-ground juxtaposition, this can be a difficult decision. Which is the "subject" here -- the tractor (foreground) or the church (background)? Which should I let go soft? And what can I do to avoid having to make the choice?

Helsinki Cathedral with tractor. (Sigma 12-24 with 10D.)

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. (Top-end cameras with high-precision AF sensors are probably noticeably better at this, but even they can't solve the problem; only mitigate it.)

In practice, this means that while you'll have no trouble focusing on subjects up to, say, 2 meters distant, beyond that, the AF will take something of a wild guess. You, however, know more than the camera: by estimating subject distance and focusing manually according to the distance scale on the lens, you can get much closer to ideal -- and the great depth of field will mask any small inaccuracy, especially stopped-down. So, here's what to do:

1. Auto-focus for close-up situationals, zone focus for the rest.
2. Stop down as far as you can. Use a tripod whenever possible: this means you'll be able to stop down to f/11 or below. Often it's better to go all the way to f/22, despite the slight diffraction-caused softening this causes. The wider the lens, the less you'll need to stop down -- but even on the widest rectilinear currently available, f/16 isn't overkill. If you don't have a tripod, use a monopod. If you don't have that, bump up the ISO up to 800 (exposure constraints permitting). The extra noise will matter far less than the better overall sharpness. If you still can't stop down, shoot the picture anyway: if the composition works, nobody will care about the technical trade-offs.
3. Identify the visual center, and focus on that. That'll be whatever the eye settles on after wandering around in the frame -- and if it's soft, the picture won't look right.
4. If you want infinity to be sharp, focus on infinity. If you're stopped down to f/16 or f/22, you have some room to pull back, but otherwise, don't trust hyperfocal shooting. Even though the calculated hyperfocal distance for the Sigma 12-24 wide-open at 12 mm is about 2 meters, that will leave infinity noticeably soggy.
5. If you have to choose, focus behind rather than in front of the subject. If you have enough resolution to outline an object, it'll be perceived as sharp. This means that you can get away with more softness in the foreground, where detail is generally bigger, than the background, where detail is small -- sometimes "infinitely" small.