Mirror lockup is one of those little things you can do to get sharper photographs. Sure, by itself you may not see a big difference, but combine it with a few other little things, and you’ll begin to notice that extra sharpness.
It makes sense to use mirror lockup when you have a somewhat controllable subject, like a landscape or wildflower. But, when you need to take shots quickly (like when you’re photographing wildlife or insects), then mirror lockup doesn’t make much sense because it only adds more time to your shots.
You should also avoid using mirror lockup when you’re photographing a bright light source (like the setting sun). After reading Canon’s white paper on the XTi, I’ve learned that this can actually damage the shutter curtains. [Read more…] about When NOT to use mirror lockup