What Exactly Is a Tilt-shift Lens?

A tilt-shift lens allows a photographer to alter the angle between the focal plane and the camera sensor. This adjustment allows compensation for the Keystone effect, which causes parallel lines in an image to converge if the camera is not perfectly level. Tilt-shift lenses can also be used to increase depth of field at wide apertures, and are extremely popular with landscape and architectural photographers. The nature of the design of tilt-shift lenses makes the implementation of autofocus mechanisms far too complicated to be practical therefore all current tilt-shift lenses are manual focus only lenses.