135 film and the 24 × 36 mm frame
1925 (Leica's 24 × 36 frame); 1934 (Kodak's daylight-loading 135 cartridge)
The 24 × 36 mm frame on 35mm film in a light-tight cartridge became the world's standard still format, and the yardstick, as 'full frame', for every digital sensor size.
How it began
The Leica made the 24 × 36 mm frame popular, but early users loaded film in the dark. In 1934 Kodak introduced the 135 cartridge: a light-tight metal cassette of 35mm film that could be loaded in daylight. It launched with the Kodak Retina, made by Kodak’s German factory.
How it works
The film is 35mm wide with perforations along both edges; each frame is 24mm tall and 36mm wide. Its diagonal, about 43.3mm, is the reference for crop factor: a sensor with half that diagonal has a crop factor of 2. A 50mm lens gives about the same view as the human eye on this format. See formats and crop factor.
Why it mattered
135 became the most widely used film format in history, carried by rangefinders, SLRs and compacts. When digital sensors were made the same size, from 2002, they were called full frame. Lens focal lengths are still compared through the 35mm frame.
Limits and what replaced it
For many uses digital sensors have replaced film, but 135 film is still manufactured and sales have grown again since the 2010s.
Today
135 film is still made and sold worldwide, and every crop factor is measured against its frame.