Cinema’s early days were straightforward. A 35mm celluloid frame was four perforations high, creating an industry standard ratio of 4:3, or 1:1.33, often shortened to 1:33. When synchronised sound arrived in 1929, dimensions expanded slightly to allow for the optical soundtrack strip, creating a 1:37 aspect ratio. Both of these were folded into one official image size, known as the Academy ratio. ref BFI
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