F-mount bayonet lens mount system, which is still current in a modified form today. The focal plane shutter, unlike other SLRs of the period which used a cloth material for the focal plane shutter design (NOTE: with this design, it was possible to burn a hole into the cloth of the shutter during mirror lock-up in bright sunlight) used titanium foil which was rated for 100,000 cycles of releases of the shutter (according to Nikon). The F was also a modular camera, in which various assemblies such as the pentaprisms, the focusing screens, the special 35mm roll film 250 exposure film back and the Speed Magny film backs (two models: one using the Polaroid 100 (now 600) type pack films; and another Speed Magny was designed for 4×5 film accessories, including Polaroid's own 4×5 instant film back). These could be fitted and removed, allowing the camera to adapt to almost any particular task. It was the first 35 mm camera offered with a successful motor drive system. Unlike most of the other manufacturers involved in 35mm camera production, the Nikon F was released with a full range of lenses from 21 mm to 1000 mm focal length. Nikon was also among the first to introduce what is commonly known today as 'mirror lenses' – lenses with Catadioptric system designs, which allowed the light path to be folded and thus yielded lens designs that were more compact than the standard telephoto designs. Subsequent top-of-the-line Nikon models carried on the F series, which has reached the F6 (although this camera has a fixed pentaprism). With the introduction and continued improvements being made in digital photography, the Nikon F6 is the last of the flagship Nikon F-line film SLRs. Canon In May 1959, the Canonflex SLR was introduced. The camera featured a quick return mirror and an automatic diaphragm, and was introduced with an interchangeable black pentaprism housing. It also featured newly developed 'R' series breech lock mount lenses. This SLR was superseded by the Canonflex RM, a fixed prism SLR which featured a built-in selenium cell meter. Later came the Canonflex R2000, with a top shutter speed of 1/2000 of a second. This model was also superseded by the Canonflex RM. In 1962, FL series lenses were introduced along with a new camera body, the Canon FX, which had a built-in CdS light meter positioned on the front left side of the