battery is used (as opposed to 7.5 frames per second with eight AA alkaline batteries). The F3H had the appearance of an F3P modified for high-speed photography; however, under the skin were some advanced techniques to achieve the speeds required, including something Nikon have described as "the camera and the motor drive predict and control each other's operations, so that certain operations overlap". The MD-4H was essentially a speeded-up MD4, with the gearing increased by 1.5 and the voltage raised to the standard, already capable, motor. There is one additional gold contact pin surrounding the rewind spigot, that joins with the electrical connection on the base of the camera. The front face of the drive features a slightly extended area with a switch that enables the option of using the drive at a constant 6 frames per second. Production was short, official reports from Nikon claiming only around 100 such cameras made, however it is likely that the real number exceeds that by many hundreds, as serial numbers in the high 800's have been noted . Serial numbers on the camera body begin with H9600..., and the front features the signature F3 logo with a H added. Instead of the depth-of-field preview button, it had an extended version of the button which became an aperture opening button for composing purposes. Metering was done at working aperture, instead of wide-open, as on other F3 bodies. The camera is also unique among all F3's in having a white film take up spool. Like the F3P, the DX-1 viewfinder could not be used on it. When fitted with the MD-4H it is also incompatible with the Magazine Back MF-4 250, Firing Rate Converter MK-1 and Data Back MF-14. It was the last specialized high speed film Nikon camera made and amazingly it remained the highest frame rate SLR (including digital) that Nikon made until the introduction of the Nikon D6, which achieves 14 frames per second with a reflex mirror. The camera is highly collectable, with limited numbers being produced and being the final in a line of the high speed cameras starting with the legendary Nikon S3M. F3AF Finally there was the rarely seen 'F3AF', a model which included an autofocus (AF) pentaprism finder (DX-1) capable of autofocusing with two special AF Nikkor lenses—the first of their kind, which included focus motors in the lens bodies. Those lenses were the AF-80mm f/2.8