professional photographers of the 1940s and 1950s time-period preferred to use hand-held meters such as the weston or ge selenium cell light meters, and others which were common during these periods. these hand-held meters did not require any batteries and provided good analog readouts of shutter speeds, apertures, asa (now referred to as 'iso') and ev (exposure value). selenium cells, however, could easily be judged for their light sensitivity by simply looking at the size of the cell's metering surface. a small surface meant it lacked low-light sensitivity. these would prove to be useless for in-camera light metering.