pentaprism and the right-hand thumb wind lever. Nikon, Canon and Yashica introduced their first SLRs in 1959 (the F, Canonflex, and Pentamatic, respectively). Through-the-lens light metering As a small matter of history, the first 35 mm camera (non-SLR) to feature through the lens light metering may have been Nikon, with a prototype rangefinder camera, the SPX. According to the website below, the camera used Nikon 'S' type rangefinder lenses. Through-the-lens light metering is also known as "behind-the-lens metering". In the SLR design scheme, there were various placements made for the metering cells, all of which used CdS (Cadmium sulfide) photocells. The cells were either located in the pentaprism housing, where they metered light transmitted through the focusing screen; underneath the reflex mirror glass itself, which was Topcon's design; or in front of the shutter mechanism, which was the design used by Canon with their Canon Pellix. Pentax was the first manufacturer to show an early prototype 35 mm behind-the-lens metering SLR camera, which was named the Pentax Spotmatic. The camera was shown at the 1960 photokina show. However, the first Through-the-lens (TTL) light metering SLR on the market was the 1963 Topcon RE Super, which had the CdS metering cell placed behind the reflex mirror. The mirror had narrow slits cut into the surface to let the light reach the cell providing average metering. Late in the following year, a production model of the Pentax Spotmatic was shown whose CdS light meter cells were on the pentaprism, reading the light off the focusing screen providing average reading, yet keeping the Spotmatic name, but now written in one word. Another clever design appeared in 1965, the Canon Pellix employing a pellicle mirror that is semi-transparent, placing the meter cell on an arm swinging into the lightpass behind the mirror for meter reading. Mamiya Sekor came out with cameras such as the Mamiya Sekor TL and various other versions. Yashica introduced the TL Super. Both of these cameras used M42 screw thread lenses as did the Pentax Spotmatic. Later on Fujica introduced their ST-701, then ST-801 and ST-901 cameras. The ST-701 was the first SLR to use a silicon cell photodiode, which was more sensitive than CdS and was immune to the memory effect that the CdS cell suffered from in bright sunlight. Gradually, other 35 mm SLR camera manufacturers changed their behind-the-lens meters from CdS cells to Silicon Diode photocells. Other