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.