With mirror lock-up the mirror (2) flips up towards (5) well before the shutter (3) opens. As
a result light no longer reaches the eyepiece (8).
Mirror Lock-Up
I would not buy a camera without the ability to lock-up the mirror
and thus remove the excess vibrations when the mirror snaps up
out of the way of the lens viewfinder. It means I have to click the
shutter, the mirror goes up, wait for the vibrations to die out, and
click it again, but it makes a real difference. Stacking focus means:
everything has to be motionless. Some cameras (like the Nikon
D810) have what is called an Electronic First Curtain, which allows
for vibrationless shooting. Highly recommended, but not always
available.