their interchangeable prism SLR cameras. Nikon's early Photomic finder utilized a cover in front of the cell which was raised and a reading was taken and the photographer would either turn the coupled shutter speed dial and/or the coupled aperture ring to center a galvanometer-based meter needle shown in the viewfinder. The disadvantage of this early Photomic prism finder was that the meter had no ON/OFF switch so the meter was constantly 'ON', thus draining battery power. A later Photomic housing had an ON/OFF switch on the Pentaprism. CdS light meters proved more sensitive to light and thus metering in available light situations was becoming more prominent and useful. Further advances in CdS sensitivity, however, were needed as CdS cells suffered from a 'memory effect'. That is, if exposed to bright sunlight, the cell would require many minutes to return to normal operation and sensitivity. Through-the-lens metering Through-the-lens metering measures the light that comes through the camera lens, thus eliminating much of the potential for error inherent in separate light meters. It is of particular advantage with long telephoto lenses, macro photography, and photomicrography. The first SLRs with through-the-lens metering were introduced by Japanese manufacturers in the early to mid-1960s. Nikon F and F2 with interchangeable photomic prisms 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. In 1972, the Nikon F2 was introduced. It had a more streamlined body, a better mirror-locking system, a top shutter speed of 1/2000 of a second and was introduced with its own proprietary, continually improving Photomic meter prism heads. This camera's construction was mechanically superior to the F, with some models using titanium for the top and bottom cover plates, and featured slower shutter speeds via the self-timer mechanism. All Nikon F and F2 Photomic prism heads coupled to the shutter speed dial of the respective camera, and also to