Mirror lockup (often abbreviated MLU) is a DSLR feature that flips the reflex mirror up in a separate, earlier action than the shutter release, allowing internal vibrations from the mirror movement to damp out before the actual exposure begins. A first press of the shutter raises the mirror; a second press, or a self-timer countdown, then fires the shutter. The result is a vibration-free exposure at the moment light hits the sensor.
The vibration MLU addresses is called mirror slap. When the reflex mirror swings up, it impacts a damper at the top of the mirror box, and the whole camera body ripples for a brief period afterward. At fast shutter speeds this is hidden by the short exposure, and at very long exposures any vibration is averaged out across seconds of integration. But in the awkward middle range, roughly 1/30 second to 2 seconds, mirror slap can blur otherwise sharp frames, particularly with telephoto lenses on a tripod.
Landscape photographers shooting at base ISO with stopped-down apertures and ND filters live in exactly this danger zone, which is why MLU became standard practice for tripod-based work with DSLRs. Macro shooters at high magnification ratio face the same problem amplified: at 1:1 or beyond, even tiny vibrations translate into visible movement on the sensor. Some camera bodies pair MLU with an electronic first curtain shutter to further reduce vibration sources, leaving only the second curtain to close mechanically.
Procedurally, the photographer composes, focuses, locks down the tripod head, then triggers the mirror with one press. A pause of two to five seconds allows the body to settle, and the exposure follows. Cable releases, wireless triggers, or the camera’s built-in self-timer eliminate finger contact that would reintroduce vibration. Many Canon, Nikon, and Pentax bodies offer an automated “mirror up plus delay” mode that performs the sequence with a single press.
Mirror lockup is entirely irrelevant on mirrorless cameras, which have no reflex mirror to begin with. On those bodies, the vibration sources that remain are the mechanical shutter curtains, addressed by electronic first curtain or fully electronic shutter modes. As the industry has migrated to mirrorless, MLU has gradually faded from working photographers’ vocabulary, surviving mostly in DSLR-focused tutorials and on older bodies still in heavy field use. Photographers transitioning to mirrorless typically find their tripod work sharper at intermediate shutter speeds without changing technique, simply because the vibration source MLU was invented to suppress no longer exists.
For owners of DSLRs still doing critical tripod work, the habit pays off. A simple test, a series of identical frames at 1/8 second on a long lens, with and without MLU, will reveal whether mirror slap is costing sharpness in any given setup. The difference is usually visible at 100 percent on screen.