I keep seeing the opinion that exact focus is essential for sharp pictures. Are the people who worry so much about exact focus shooting copies of paintings? Do the people who reiterate this point know about depth of field? I read calls for precise focus and I also read concern about diffraction loss. Yes, precise focus usually is necessary, and diffraction loss is something to avoid if you can, but most of the time you're shooting things that need more than a couple inches of DOF, and a very small amount of sharpening will eliminate diffraction loss.
I think Hans is right. Most of the time the real problem is camera shake. There are plenty of situations where tack-sharpness simply isn't critical. Cartier-Bresson's early photographs are good examples. But if you want to do Ansel Adams you simply must have a solid tripod and a (nowadays electric) cable release. You need to shelter the tripod from the wind. You need to use mirror-up with a delay. You need to focus accurately -- somewhere -- if you're shooting landscape, usually about a third of the way to the horizon, but you also need to use a reasonable aperture setting so that things other than the focal point will be in focus. Yes, f/16 or f/22 will cause some loss of sharpness from diffraction, but at least everything that should be in focus in the picture will be sharp (if you use tripod, cable release, mirror up with a few second pause before you trip the shutter). You can take care of the diffraction loss in post-processing, but if part of your picture is out of focus because of shallow DOF you're out of luck.