Conceptually intermediate between the Rangefinder Leicas and the SLR Leicas was the Leica Visoflex System, a mirror reflex box that attached to the lens mount of Leica rangefinders (separate versions were made for the screwmount and M series bodies) and accepted lenses made especially for the Visoflex System. Rather than using the camera's rangefinder, focusing was accomplished via a groundglass screen. A coupling released both mirror and shutter to make the exposure. Camera rangefinders are inherently limited in their ability to accurately focus long focal-length lenses and the mirror reflex box permitted much longer length lenses. Throughout its history, Leitz has been responsible for numerous optical innovations, such as aspherical production lenses, multicoated lenses, and rare earth lenses.