Johnson Standard Exposure Calculator. This was the earlier form of the “Standard” calculator, dating from the early 1950s
Circa 1950

Before exposure meters became the norm, a wide range of calculators were available to enable the correct exposure to be determined in both daylight and artificial light. Some were film or camera makes specific, such as those by Kodak, while others were generally applicable, such as the range produced by Johnson of Hendon in the UK.
In late 1946 (RH), Johnson & Sons, Manufacturing Chemists Ltd, began marketing a ‘British Standard’ (to BS 935/1941) 6.5cm diameter rotary exposure calculator made of ‘ivorine’. This plastic simulates the appearance of ivory. It was reviewed in the 4th December 1946 issue of the British Journal of Photography (BJP) on p132. “Packed in a stout envelope, with full instructions, the price is 4s/2d (21p) including Purchase Tax” (PT). The full review and the Johnsons’ advert in the 1947 BJP Almanack are viewable here. 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, with a printed date of 1968. Construction is the same as for the colour above. The filter factor was set against ‘start’ in using first, and 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 were successively dialled. Then, the shutter speed on the calculator’s front could be read off against the chosen aperture.
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.