Light-value system
The Light-value system is a device, usually found on leaf-shutter cameras of the late 1950s, that couples the speed and aperture rings. It allows to change 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 Light-value scale, that you could directly transfer to a Light value ring, thus setting the correct exposure.
The inconvenience is that you must unlock the device to change only one of the two parameters, or simply to change the exposure. This system was made outdated by the direct coupling of the camera's internal exposure meter to the aperture and speed controls. The need for a separate Light-value scale had disappeared, and the ability to switch the aperture and speed together was only retained in some cameras having both rings placed side by side and turning in the same direction, so that they can be turned together when grabbing both of them.
