(one 4LR44 or PX-28) microprocessor-controlled manual-focus SLR with manual exposure control or shutter priority, aperture priority or programmed autoexposure. A fifth mode is "stopped down AE", in which the aperture is closed and alterable by the photographer and the camera selects the shutter speed based on the actual light reading. This differs from aperture priority in which the aperture is not closed until a photograph is taken and the shutter speed is calculated based on the light measured through the fully open aperture. Stopped down AE existed so that old FL lenses could be used with at least some kind of AE, and was also useful for photomicroscopy, manual-aperture lenses, etc. The A-1 is the first SLR to have all four of the now standard PASM exposure modes. It has a viewfinder exposure information system using a six-digit, seven-segment per digit, red alphanumeric LED display on the bottom of the viewfinder to indicate the readings of the built-in silicon photocell light meter. It supports one metering mode, centerweighted average metering. The focusing screen also has Canon's standard split image rangefinder and microprism collar focusing help. Design history Beginning with the amateur level Canon AE-1 of 1976, there was a complete overhaul of the entire Canon SLR line. The 1970s and 1980s were an era of intense competition among the major SLR brands: Canon, Nikon, Minolta, Pentax and Olympus. Between 1975 and 1985, there was a dramatic shift away from heavy all-metal manual mechanical camera bodies to much more compact bodies with integrated circuit (IC) electronic automation. In addition, because of rapid advances in electronics, the brands continually leapfrogged each other with models having new or more automatic features, and less expensive components and assembly. The industry was trying to expand out from the saturated high-end professional market and appeal to the large mass of low-end amateur photographers keen to move up from compact automatic leaf shutter rangefinder cameras to the more "glamorous" SLR but were intimidated by the need to learn all the details of operating a traditional SLR. The A-1 is the high technology standard bearer of the landmark Canon amateur level A-series SLRs. The other members of the A-series are the Canon AE-1 (released 1976), AT-1 (1977), AV-1 (1979), AE-1 Program (1981) and AL-1 (1982). They all use the same compact aluminum alloy chassis, but with differing feature levels and outer cosmetic acrylonitrile-butadiene-styrene (ABS) plastic panels. By sharing