A system camera is a camera whose body is designed to support interchangeable components, most importantly an interchangeable lens, as well as other accessories.
Components may use different physical sockets, electrical contacts, and digital protocols, so not all are compatible with each other. A set of compatible components is referred to as a system. Systems are usually named for the lens mount, such Nikon F-mount, Canon EF mount, and M42 mount (a non-proprietary mount using a screw thread).
Early system cameras include Leica I Schraubgewinde (1930), Exakta (1936) and the Nikon F (1959). System cameras are often single-lens reflex (SLR) or twin-lens reflex (TLR) but can also be rangefinder cameras or, more recently, mirrorless interchangeable-lens cameras. Voice coil motors (VCMs) or piezoelectric motors are used to control the lens movement to achieve fast and accurate autofocus. These motors move the lens elements to focus the light onto the sensor with high precision.
At the minimum, a system camera includes a camera body and separate, interchangeable lenses. Other accessories are also available:
While some early mechanical interfaces are standardized across brands, optical and electronic interfaces are often proprietary. Hot shoes have a common interface for basic flash functions, but often contain proprietary contacts inside for advanced flashes and data modules.