A mirrorless camera is a camera that does not have a mirror. Though most cameras, including those used in smartphones, lack mirrors, the term mirrorless is primarily used to describe digital interchangeable lens cameras, in order to distinguish them from DSLRs, which have historically dominated interchangeable lens cameras. Lacking a mirror system allows the camera to be smaller, quieter, and lighter.
In cameras with mirrors, light from the lens is directed to either the image sensor or the viewfinder. This is done using a mechanical movable mirror which sits behind the lens. By contrast, in a mirrorless camera, the lens always shines light onto the image sensor, and what the camera sees is displayed on a screen for the photographer. Some mirrorless cameras also simulate a traditional viewfinder using a small screen, known as an electronic viewfinder.
DSLRs can act like mirrorless cameras if they have a "live view" mode, in which the mirror moves out of the way so the lens can always shine onto the image sensor.
Many mirrorless cameras retain a mechanical shutter. Like a DSLR, a mirrorless camera accepts interchangeable lenses. Mirrorless cameras necessarily have worse battery life because they need to power the screen and sensor at all times.
Terminology
Mirrorless cameras are sometimes referred to as mirrorless interchangeable-lens cameras (MILC), or digital single-lens mirrorless (DSLM) cameras. This latter name highlights their connection to DSLRs. Other terms include electronic viewfinder interchangeable lens (EVIL) cameras and compact system cameras (CSCs).
Among these, mirrorless interchangeable-lens camera, or just mirrorless, is the most popular. This term sometimes sparks debates, as to what does and does not count as a mirrorless camera, with some saying that cameras such as film rangefinders count as mirrorless, and others saying that old cameras cannot be retconned to be mirrorless.
Overview
Mirrorless cameras are mechanically simpler than DSLR cameras, and are smaller, lighter, and quieter due to the elimination of the moving mirror. While nearly all mirrorless cameras have a mechanical shutter, many also have an electronic shutter, allowing completely silent operation.
As the image from the lens is always projected onto the image sensor, features can be available which are only possible in DSLRs when the mirror is locked up into "live view" mode. This includes the ability to show a focus-peaking display, zebra patterning, and face or eye tracking. The electronic viewfinder can provide live depth of field preview, can show a poorly-illuminated subject how it would look with correct exposure in real time, and makes it easier to view the results of an exposure in bright sunlight.
With the latest phase-detect autofocus available on some mirrorless cameras, the autofocus speed and accuracy of some models has been shown to be as good as DSLRs. But mirrorless cameras have shorter battery life than DSLRs due to prolonged use of LCD and/or OLED viewfinder displays, and often smaller buffers (to save battery). On-sensor autofocus is free of the adjustment requirements of the indirect focusing system of the DSLR (which relies on a separate autofocus sensor located below the reflex mirror), and as of 2018 mirrorless cameras could shoot with phase-detect autofocus at up to 20 frames per second using up to 693 focus points—a number far exceeding what was available on any DSLR. However, on-sensor phase detection autofocus (except for Canon's Dual Pixel Autofocus) repurposes pixel sites for autofocus acquisition, so that image data is partially or entirely missing for the autofocus "pixels", which can cause banding artifacts in the final image.
History
Mirrorless cameras have technically existed since the invention of the camera. However, the term mirrorless came into use in order to describe Micro Four Thirds cameras when they were announced in 2008, especially as the first Micro Four Thirds camera, the Lumix G1, was designed to be as similar to a DSLR as possible. There are other terms that were created, too, but mirrorless became the most popular.
Until the mid 2010s, mirrorless cameras were dismissed by many photographers, because of their laggy and low resolution screens, when compared with the clarity and responsiveness of the optical viewfinders used on DSLRs, especially under strong sunlight or when photographing the sky at night. In addition, mirrorless cameras were known for having worse autofocus performance compared to DSLRs, and much worse battery life.