Starting in 1976, Nippon Kogaku introduced the Nikon Speedlight SB-2 (guide number 82/25 (feet/meters) at ASA 100), SB-5 (guide number 105/32 (feet/meters) at ASA 100), SB-6 (guide number 148/45 (feet/meters) at ASA 100) and SB-7E (guide number 82/25 (feet/meters) at ASA 100) electronic flashes. Note that the F2 did not use a standard ISO hot shoe to mount flash units. Instead, the SB-2, -6 and -7E mounted in a unique-to-Nikon-F-and-F2 hot shoe surrounding the film rewind crank. Manually rewinding film could not be done with a flash mounted in this shoe because the flash blocked the crank. Standard ISO foot flashes can be connected to the Nikon shoe via the Nikon AS-1 Flash Unit Coupler.