mount system for a quarter century. It was used by almost two dozen different SLR brands, most notably Asahi Pentax. (Asahi became so closely associated with this mount that it was, and still is, often erroneously referred to as the Pentax screw mount.Schneider, "Camera Collector: screw-mount SLR saga, part 1," pp. 20, 23, 26) However, by the early 1970s, the M42's limitations, especially no provision for auto-diaphragm lens open aperture viewing and metering, were becoming serious liabilities. After unpopular and uncoordinated attempts to modify the screw mount to support auto-diaphragm lenses with open aperture metering, Asahi abandoned the M42 screw mount in 1975, effectively ending production of this lens mounting system. 1971 Fujica ST701 (Japan): first SLR with silicon photodiode light meter sensors.Mantale, p. 163 Early SLR TTL meters used cadmium sulfide (CdS) cells (see Topcon RE Super and Asahi Pentax Spotmatic above), as they were the first sensors small enough to be internally mounted. However, CdS needed fairly bright light and suffered from a "memory" effect where it might take 30 seconds or more to respond to a light level change. Although silicon's infrared response required blue filtration to match the eye's spectral response, silicon supplanted CdS by the late 1970s because of its greater sensitivity and instantaneous response. 1972 Fujica ST801 (Japan): first SLR with viewfinder light emitting diodes.Matanle, p. 163 Had a seven LED dot scale to indicate extreme overexposure, +1 EV, +½ EV, 0 (correct exposure), –½ EV, −1 EV, extreme underexposure readings of its silicon photodiode light meter, instead of the traditional but delicate galvanometer needle pointer. A sister camera, the Fujica ST901 (Japan) of 1974, was the first SLR with a viewfinder LED digital data display.Lea, p. 93 It had calculator-style LEDs showing camera's aperture priority autoexposure set shutter speeds from 20 to 1/1000 second in 14 nonstandard steps. Although they were replaced by more energy efficient and informative LCDs in the 1980s (see Nikon F3, below), the use of LEDs in the ST801/ST901 were major steps in the escalation of electronics in 1970s camera design 1972 Olympus OM-1 (Japan): first compact full-featured 35 mm SLR. At 83×136×50 mm and 510 g, it was about two-thirds the size and weight of most earlier 35 mm SLRs."Modern Tests: Olympus OM-1: World's Smallest 24×36mm SLR," pp. 98–100. Modern Photography, Volume 37, Number 4; April 1973. Wade, Short History. pp. 125–126 Excellent mechanical design with excellent interchangeable