A definition of Mirror Up/Lock

A function which raises the mirror of a DSLR in a separate action to the shutter being tripped. Ordinarily these two actions happen as shutter opens, although the movement of the mirror can cause vibrations which introduce camera shake. Raising the mirror first means that its movements do not stand to affect the image as it’s being taken, although in this position it does prevent an image of the scene from reaching the viewfinder.