This 0.71x Lens Mount Adapter from Viltrox allows a full-frame Canon EF-mount lens to be mounted onto a Canon EF-M-mount camera. While doing so, the adapter’s optics help to maximize the connected lens’s apparent angle of view while providing a one-stop increase for the camera over the lens’s aperture range.
If a lens intended for 35mm film or similarly-sized full-frame camera sensors is connected to a Canon EF-M-mount camera, it would project an image larger than the camera’s APS-C sized sensor was built to accept. Able to capture only the center portion of the projected image, the effective angle of view of the lens would be narrowed, creating an in-camera “crop factor.” This adapter condenses the light from a lens designed for 35mm film or “full frame” camera sensors and projects it onto a Canon EF-M-mount camera’s APS-C sensor. This allows the APS-C sensor to utilize more of the light gathered by a connected lens, which effectively reduces the crop factor while making available to the sensor an additional one stop of light for each of the lens’s f/stop settings.