also known as 35mm
In the context of cameras, the most popular film format of all. It is about 35mm wide - a good part of it is used by sprocket holes.
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O O O O O O O O O O O O O O O O
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}- area usable for image, about 24 mm
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O O O O O O O O O O O O O O O O <--- sprocket holes (I really like the word sprocket)
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Widespread use begins with the Leica camera family. It is also used as motion picture stock.
On 135 film, the standard choice is shooting 24x36 (in millimeters) images, but there is also 24x18 (known as half-format) and some panoramic image sizes, like 24x52 and others. The film itself imposes no horizontal size.