Mirror lock-up (often abbreviated to MLU) is a feature employed in many Single Lens Reflex (SLR) cameras. It allows the operator to reduce

vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic function, periodic, such as the motion of a pendulum ...

-induced

motion blur Motion blur is the apparent streaking of moving objects in a photograph or a sequence of frames, such as a film or animation. It results when the image being recorded changes during the recording of a single exposure, due to rapid movement or lo ...

during exposure. It also allows the mounting of lenses which extend into the SLR's mirror box when mounted.

Reducing vibration

Normal operation in an SLR camera involves flipping the mirror up out of the light-path just before the shutter opens, and then returning it when the shutter closes (although very early SLR's required the shutter to be cocked for the mirror to return). This causes vibration of the camera, particularly when the mirror slaps into the top of the mirror box. This vibration quickly dies away so the most motion blur is actually seen with short shutter times that capture multiple 'swings' of the vibration (shutter speeds of 1/2 to 1/60 second are often affected by this). While longer exposures will capture all of the vibrations, the exposure will be dominated by light captured when the camera is vibration-free (assuming a steady mount). Mirror lock-up involves flipping the mirror up well before the shutter opens, allowing the vibrations to die down before exposing the film. On some cameras MLU may be operated by an extra push of the shutter button, the second push resulting in the actual opening of the shutter. Other arrangements may involve an extra lever or button that flips the mirror up before using the shutter release button normally. On some cameras MLU is not a separate feature, but operated as part of the

self timer A self timer is a device on a camera that gives a delay between pressing the shutter release and the shutter's firing. It is most commonly used to let the photographer to take a photo of themselves (often with a group of other people), hence t ...

(notably the Nikon FE line of cameras). Note that when the mirror is in the up and locked position, the subject is no longer visible through the

viewfinder In photography, a viewfinder is what the photographer looks through to compose, and, in many cases, to focus the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main ...

. Therefore, the photographer must compose the photograph prior to activating mirror lock-up and keep the camera from moving. Use of a

tripod A tripod is a portable three-legged frame or stand, used as a platform for supporting the weight and maintaining the stability of some other object. The three-legged (triangular stance) design provides good stability against gravitational loads ...

helps prevent movement of the camera during this operation. Combined with a remote or cable release, this greatly reduces the potential for vibration of the camera.

Lenses requiring mirror lock-up

Some lenses are designed such that they extend into the mirror box when properly mounted on an SLR. These may include early wide-angle lenses for SLR's, certain lenses designed for

rangefinder camera A rangefinder camera is a camera fitted with a rangefinder, typically a split-image rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus. Most va ...

s, and certain other non-SLR lenses adapted for SLR use. Mirror lock-up must be activated prior to mounting such lenses, and kept in place in order to prevent damage to the lens or camera. This method of operation prevents any use of the SLR viewfinder with the lens. Prior to the development of lenses utilizing the

Angénieux retrofocus The Angénieux retrofocus photographic lens is a wide-angle lens design that uses an inverted telephoto configuration. The popularity of this lens design made the name retrofocus synonymous with this type of lens. The Angénieux retrofocus for s ...

concept, mirror lock-up was essential to wide-angle SLR photography.

References

{{reflist

External links

Understanding Mirror Lock-Up
on The Luminous Landscape
Mirror Lock Up
by Bob Atkins on Photo.net Photography equipment