Nominal film names originated as marketing conveniences during the early twentieth century. Roll film sizes needed simple retail categorizations so hobbyists and portrait studios could buy emulsion stocks without carrying fractional metric charts. The designation "6x4.5 cm" (often called 645) implies an image area of 60 mm by 45 mm. In reality, modern 120 film sits inside an exposure gate bounded strictly by the camera's internal chassis rails.
Across nearly all Japanese and European camera systems, the maximum exposed width across the film strip is limited to 56 mm. This restriction ensures that the exposed image stays comfortably within the paper backing borders, preventing edge fogging and light leaks along the spool rim. When paired with a 645 gate, the frame height cuts off around 41.5 mm to 42.5 mm.
The resulting 56x41.5 mm actual measurement creates an aspect ratio of roughly 1.35:1, matching the traditional 4:3 display standard far closer than a mathematically pure 60x45 mm rectangle (which would sit at 1.33:1). Photographers sizing mats for fine-art exhibition prints routinely miscalculate border crops when relying on textbook labels instead of raw physical measurements.
