A 35mm movie frame size measured 18x24mm, with the film running vertically through the camera. But Barnack ran film through his prototype horizontally, doubling one of the frame dimensions. So was born the 24x36mm negative size that became the standard for 35mm still photography. The year was 1913, one year before the outbreak of World War I in Europe.
Following the war, Barnack returned to his project, modified that first prototype and persuaded Ernst Leitz, head of the company, to build a batch of cameras to test the market.
These cameras, known as the Model 0, were similar to the production models that followed, apart from using a fold-up viewfinder in place of the subsequent optical type. Also, shutter speeds were not designated in fractions of a second, but rather as the width of the slit in the focal plane shutter. The camera used a Leitz 50mm Anastigmat lens.
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