The IIIf was introduced in 1950 as Leica's first production originally equipped flash sync capable body and quickly became Leica's highest selling Screw Mount Camera body. Not exactly revolutionary in design it was little more than a IIIc body with flash sync and terminal.
At serial number 611000 the shutter mechanism was redesigned and went to a lighter, faster mechanism. At almost the same time the shutter speed sequence changed and the markings for delay chosen marked on the top plate surrounding the shutter speed dial change from BD to RD. (Page 67 of Delaney's Collectors Guide shows an f with BD and the newer shutter speed sequence and with a 611,337 serial number). Below, please find examples of BD and RD models with the normally associated shutter speed sequences.



The flash sync (unlike later electronic flashes) is a non trivial issue. Bulbs take a finite interval to come up to temperature and render a uniform even burn. A short but important delay is involved in this warm up. The red or black ring allows the precise setting of the delay for bulb warm up to optimize the use of flash at various shutter speeds.
During the 1954 production year the IIIf RD ST model was introduced and was Leica's first self timer equipped body. Earlier produced bodies could be returned to the Leica factory for the addition of this feature.