	The vast majority of SLRs now sold are digital models, even though their size, form factor, and other design elements remain derived from their 35 mm film predecessors. Whether a dedicated digital design such as the Olympus Four-Thirds system, which permits equivalent performance with smaller and lighter cameras, will ultimately supersede the film-derived designs from Canon, Nikon, Pentax, and Sony is as yet unclear. Additionally SLRs are facing a threat from the rapidly expanding mirrorless interchangeable-lens camera segment among all types of camera user.