Angle Finders A2 and B could also be attached to allow critical focusing and / or waist-level use (if one did not want to fit either the Speed Finder or Waist Level Finder). Flash accessories With a removable viewfinder, the F-1's flash coupler originally attached atop the rewind crank. Initially, there were two flash couplers, D and L. The D model was a simple x-synch coupler that allowed any non-dedicated manual or auto flash to be used. The Flash Coupler L contained two batteries (now hard to find, one being originally a 1.35v mercuric oxide and the other being the uncommon PX-1 size), one which powered a light to light up the metering window visible in the viewfinder, and the other to work with the original Canon Auto tuning system (CATS). The CATS used a special auto flash, the SpeedLite 133D and Flash auto Rings A, B, A2 and B2 and Canon 50 mm and 35 mm Lenses which signalled through the cords the distance of the subject and the charge level of the flash to allow match needle flash photography. Canon announced and produced manuals for a high power handle mounted ("pototao masher") flash designated SpeedLight 500A. This was also to use the CATS equipment It appears in some Canon publications, and there are user instructions for it, however most people have never actually seen a SpeedLight 500A. For low ambient light photography without flash, Canon provided the Finder Illuminator F which slid over the same flash contacts at the flash couplers. It contained a small battery powered light to light up the metering window. Later, Canon introduced the Flash Coupler F, which fastened over the Eye Level Finder, making the camera look like more like one with a fixed viewfinder and hot shoe. This flash coupler, obviously could not be used with any other viewfinder, and did not have the electronics that the L model had, but it was more compact and the newer A-series flashes which had auto flash capabilities had now superseded the old SpeedLite 133D. The CATS flash equipment was for the later electronic Canon F1 New (1981) and it allowed aperture settings from the camera to be communicated to the flash unit. It was also possible for the flash unit to select an appropriate camera aperture based on its own photo cell exposure reading, provided that the motor drive was also present. The motor