In the days before exposure meters became the norm, a whole range of calculators were available to enable the correct exposure to be determined in both daylight and artificial light. Some were film or camera make specific, such as those by Kodak, whilst others were generally applicable such as the range made by Johnson of Hendon in the UK.
Johnson Standard Exposure Calculator for Black & White & Colour Negative Films in Daylight. Another calculator from the range of Johnsons of Hendon Ltd. It also has its paper envelope and instruction sheet, the latter with a printing date of 1968. Construction is the same as for the Colour one above. In use first the filter factor was set against ‘start’, then the subject group was selected and dialled to ‘stop’. Then the calculator was turned over and the light value, the film speed and weather conditions successively dialled. Then on the front of the calculator the shutter speed could be read off against the chosen aperture.
Johnson Standard Exposure Calculator. This was the earlier form of the “Standard” calculator, dating from the early 1950s. Its construction is the same as the Flash calculator. It is made of Ivorine (Celluloid). Its operation was similar to that of the later model but using Time rather than Light Value.