Light-value system
The Light-value system is a mechanism, usually found on the lens barrels of leaf-shutter cameras of the late 1950s, that couples the speed and aperture rings. It allows choice among the various aperture / speed combinations that result in the same exposure by a single twist of both rings together, with no need for separate adjustments of aperture and speed. It was meant to be used together with exposure meters giving a reading in the Exposure-Value (EV) scale, that can be directly transferred to an EV (sometimes confusingly called LV) dial on the lens, thus setting the correct exposure.
This system was made obsolete by the direct coupling of the camera's internal exposure meter to the aperture and speed controls. The ability to switch the aperture and speed together was only retained on some cameras' lens barrels having the two rings side by side and with equidistant stops, such that the two rings can easily be turned together if and when desired.
