Before there were standards
The accepted answer's starting point is that formats were originally arbitrary — a manufacturer's new camera design simply used whatever format that design called for. The confusion this produced was addressed by numbering the formats from a fixed starting point in the early twentieth century, and two of those numbers survive as the names of film stocks still used today. The shapes were not chosen for how they look.
What 35mm decided
- Many current ratios derive from the physical geometry of 35mm film: its width, and how many perforations tall each frame is.
- Sound is part of the explanation. Once a soundtrack had to occupy space on the strip, the picture area available changed shape, and ratios followed.
- This is why the same numbers recur across still photography and cinema — both inherited the same strip of film.
The much earlier inheritance
One answer traces the lineage further back than film. The first photograph measured roughly six and a half by eight inches, and the plate sizes of the first commercially successful process became an international standard — whole, half and quarter plates, with proportions close to the ratios still recognisable now. Photography standardised its frame shapes before it had any technical reason to prefer one.
Why this is worth knowing
- None of the common ratios is optimal for anything. They are the residue of manufacturing decisions and a numbering scheme.
- Which means treating them as a constraint rather than a convention is a choice, not a requirement.
- It also explains why cinema accumulated so many competing widescreen formats: each was an attempt to get a different shape out of the same physical strip.