This is a great function to employ when using tripod-necessary shutter speeds. Mirror lockup greatly reduces camera shake due to shutter slap (vibrations caused by the mirror flipping up and hitting the foam stop) when shooting with super-telephoto or close-up macro lenses. Shutter speeds of 1/15 and 1/8 sec. are especially susceptible to this phenomenon because they're both not long enough to diminish the vibration blur over time and not short enough to arrest it. It's highly recommended to activate this function when you're shooting slow-shutter-speed images or when you're not using the viewfinder to compose and shoot close-ups.
During mirror lockup, never point the camera toward the sun, as this can damage the shutter curtains.
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