lenses, with certain limitations or exceptions. Full compatibility requires lenses that support the Automatic Maximum-Aperture Indexing (AI) specification. This includes most Nikon lenses manufactured after 1977. Pre-AI lenses can be used, but only with stop-down metering. Many newer Nikon and third-party F-mount lenses are also compatible. The only major exceptions are G-type Nikkor lenses, which have no aperture ring and thus no way of properly controlling exposure, and DX Nikkors, which do not resolve an image large enough to cover the 135 frame. All other AF Nikkor lenses will mount and be usable, but autofocus and Vibration Reduction (VR) will not be supported. Both IX Nikkor lenses, for Nikon's Advanced Photo System (APS) SLRs, and very old "invasive" Fisheye-Nikkor lenses from the 1960s must not be mounted on the FM, as their rear elements will damage the FM's reflex mirror. Other features The FM has a "full information" viewfinder. In addition to the metering LEDs; the viewfinder also displays the set shutter speed and lens aperture to give context to the LEDs. A fixed K-type focus screen with 3 mm split-image rangefinder and 1 mm microprism collar is fitted. Major system accessories for the FM include the MD-11 and MD-12 motor drives; which enables continuous film advance at 3.5 frames per second, the Data Back MF-12; enabling the imprinting of date and time data on the film, and the Speedlight SB-8E electronic flash; guide number 82/25 (feet/meters) at ASA 100. Note that there were two distinct versions of the FM, depending on how the FM's light meter (see below) was activated when a motor drive was mounted. The original FM had a rotary switch shutter button collar to lock the shutter, or switch to motor drive mode when the MD-11 was mounted. The MD-12 activated the meter automatically and the switch was omitted on late (1979 or after) FMs. Nippon Kogaku also took the opportunity to construct late FMs more strongly internally than early FMs. Design history Beginning in 1977 with the FM, there was a complete overhaul of Nippon Kogaku's entire Nikon SLR line. The 1970s and 1980s were an era of intense competition between the major SLR brands: Nikon, Canon, Minolta, Pentax and Olympus. Between circa 1975 to 1985, there was a dramatic shift away from heavy all-metal manual mechanical camera bodies to much more compact bodies with microprocessor electronic automation. In addition, because of rapid advances in