The Blackmagic URSA Mini Pro EF Mount works with the interchangeable lens mount on URSA Mini Pro and URSA 4K. The URSA Mini Pro EF Mount with Shims is a replacement/spare mount for the URSA Mini Pro and URSA 4K as they come with an EF mount installed. The Blackmagic URSA Mini Pro EF Mount lets you use high quality Canon compatible photo lenses like Zeiss, Sigma and more. The mount incorporates the electronic connections that allow the lens to work with the camera. The included set of shims enables precise flange-distance adjustment when installing this mount on your camera.