A mirrorless camera is an interchangeable lens camera that omits the reflex mirror and pentaprism found in DSLRs. Light passes directly from the lens to the imaging sensor at all times, and the sensor feeds a live signal to an electronic viewfinder and rear LCD continuously. Composition, focus, metering, and exposure are all performed off the sensor itself rather than off a secondary AF/AE module fed by the mirror.
Removing the mirror box has cascading design consequences. The lens flange (the distance from the lens mount to the sensor) can be much shorter, typically 16 to 20 mm versus 44 to 46 mm on DSLRs. This shorter flange enables more compact bodies and, crucially, allows the optical designer more freedom in the rear of the lens, particularly for wide angles, where retrofocus compromises were a constant on SLR designs. It also lets older lenses from longer-flange systems be adapted via simple spacer adapters, a major reason mirrorless drew early enthusiast attention.
Phase detection autofocus moved onto the sensor itself, with dedicated phase-detect pixels distributed across most or all of the frame. This delivered AF coverage that DSLRs could only dream of, since their phase-detect array was confined to a small central rectangle. On-sensor phase detect, combined with eye detection AF and machine learning subject recognition, has made consumer-grade mirrorless cameras outperform professional DSLRs of just a few years prior at tasks like tracking faces and animals.
Exposure simulation is another mirrorless advantage. The EVF previews the final exposure, white balance, and even applied film simulations or LUTs in real time, eliminating the chimping cycle of shoot-check-adjust that defined DSLR shooting. Beginners learn the exposure triangle faster because they see the consequences immediately. Downsides include EVF lag in early implementations, higher battery drain (the sensor and EVF run continuously), and a learning curve when using strobes, since the EVF cannot preview flash exposure.
The lineage runs from the 2008 Panasonic Lumix G1 (the first Micro Four Thirds body) through the Sony NEX series, the Fujifilm X-Pro1, Olympus OM-D, and finally the migration of Canon, Nikon, and Pentax/Panasonic into full-frame mirrorless with the EOS R, Nikon Z, Lumix S, and Pentax K-mount-adjacent efforts. By the early 2020s, mirrorless had become the dominant interchangeable lens camera architecture, and most flagship lens releases targeted mirrorless mounts first or exclusively.
Mirrorless also unlocked the electronic shutter as a first-class option, with stacked sensors enabling fast readout to minimize rolling shutter and approach global shutter behavior. IBIS spread from Olympus and Sony to nearly every full-frame system. Video features (10-bit codecs, log gamma, 4K and 6K capture, internal RAW recording) became standard. The architectural change was not just about removing a mirror; it rewrote what a stills camera could be.