the nikon f, was delivered since 1962 with various pentaprism metering heads. the photomic series of prisms, which was initially designed with a direct coupled-metering cds photocell (2 models were produced). the photomic prism head later evolved to include the photomic t with ttl in 1965, a behind-the-lens metering prism head which metered an averaging pattern of the focusing screen. the later center-area reading photomic tn, concentrated 60% of its sensitivity in the central portion of the focusing screen and the remaining 40% for the outlying screen area. the photomic ftn was the last of the photomic finders for the nikon f.