This page is no longer actively maintained. (Pardon?)
Seems like they don't make 'em like they used to. FED's website says they quit camera production in 1997. Pending a proper article, here's how to use the FED-5's light meter.
This is the Industar-61L/D, the FED-5's standard lens. The L stands for lanthane, a constituent of the optical glass. (For more about lanthane's apparent radioactivity, see my FED-5 Radioactivity page.) The D stands for "dalnomernaya", which means "rangefinder type". The Z (as found on for instance the Industar-61L/Z macro lens in M42 mount) stands for "zerkalnaya", which means "mirror type", i.e. suited for SLRs. More I61 info on Bob Monaghan's site.