Mirror lock-up is a feature found on some SLR camera models that allows the user to lock the mirror, which is located in front of the sensor or film, in an upward position before taking a photograph. The main purpose of this feature is to reduce camera vibrations caused by the movement of the mirror, which can result in blurry images, especially when using long exposures or working with telephoto lenses.
When you activate mirror lock-up, the mirror flips up out of the way, allowing light to hit the sensor directly. This is followed by a brief delay (usually user adjustable) before the shutter is released. The delay allows any vibrations caused by the mirror movement to dissipate before the photo is taken.
Mirror lock-up is especially useful when shooting at slow shutter speeds and/or when using long focal length lenses or teleconverters. Another common use for mirror lock-up is macro photography, especially when using high magnification in conjunction with a tripod or other stabilising equipment. It’s also helpful when making long exposure shots and for reducing camera shake when using a lens with a narrow depth of field.
It is important to note that mirror lock-up is not necessary in all situations, and can be skipped in most cases. Furthermore, if you’re shooting a dynamic scene, the delay mirror lock-up causes might interfere with your shoot. Put shortly, it’s a feature that can be useful in specific cases where the conditions require it.