Like several of the earlier versions it did feature a built in light meter. Rollei's light meter supplier was Gossen. They may or may not have a light meter. Exposure meter calibration is discussed in detail in the Light meter article. A side-effect of the FT's light meter was a dimmer viewfinder. PX675 type mercury batteries are only needed for the light meter. The Zorki 10 is a fixed-lens Zorki coupled rangefinder camera with an integrated light meter. On the x-axis of the graph is the distance the light meter moved. This allows a light meter to be used to estimate an exposure. See also "Gray card" and "Light meter". Calibration in fc or lux, is the second most important feature of a light meter. The FT was Nippon Kogaku's first SLR with a built-in TTL light meter. Prior to this, TTL metering required a special prism viewfinder with a built-in light meter. The release button activated the light meter, when slightly depressed. Gone was the cumbersome selenium light meter, replaced with a now standard internal CdS meter.