Most SLR cameras make use of a mirror that sits in front of the film or sensor, allowing you to see through the viewfinder exactly what the camera sees. When you depress the shutter button, the mirror swings up out of the way before the shutter opens.

This action of the mirror causes a small amount of vibration, sometimes called mirror slap - because it occurs when the mirror hits the buffers at the end of its motion.

Although in most photographs it is hardly noticeable, sometimes it can cause a small amount of blur - especially in macro photography. Therefore some SLR cameras offer mirror lockup - the ability to raise and lock the mirror in the open position before taking the photograph.