Zeiss M-mount lenses were originally designed to be used with 35mm film rangefinder cameras. The rangefinder camera predates the single lens reflex camera (SLR), and features a mirrorless design. This enables rangefinder lenses to be compact in size, making them great for use with mirrorless digital cameras such as the Sony A7s series.
Zeiss's M-mount lenses also make a great companion to RED cinema cameras, as RED offers an M mount. A RED DSMC camera and a Zeiss M-mount lens are the perfect pair for gimbal and Steadicam work.