Mirror lock is a camera feature that allows the viewing mirror to be locked in the up position while the shutter is fired. This prevents the mirror from moving during exposure, significantly reducing vibration and ensuring sharper images, especially at slow shutter speeds or when using long lenses.
Mirror lock is more commonly found on medium format cameras, where the larger mirror can cause noticeable vibration as it flips out of the way of the film or sensor plane. It is also useful for high-speed photography with ultra-fast motor drives, giving photographers greater control over image stability and clarity.