SUMMARY
The 24x36mm full-frame sensor size originated from Oskar Barnack's 1913 design work at Ernst Leitz. Barnack adapted silent movie film, using its 24.89mm aperture width as the height for still photographs and doubling movie frames for the length. While the Leica I was presented in 1925, the Ur-Leica prototype's design is dated to 1913, with a functional camera by 1914. Barnack's preference for a 2:3 proportion influenced the final dimensions, a choice that has defined still photography for over a century.
TAKEAWAYS
Full frame sensor size is based on silent movie film measurements.
Oskar Barnack designed the 24x36mm format at Ernst Leitz around 1913.
The format uses rotated 35mm film, with the height derived from cinema aperture width.
Barnack's preference for a 2:3 proportion influenced the final dimensions.