‘Focal length’ is typically used to indicate the angle-of-view (primes) or range of angles of view (zooms) produced by a lens. Longer focal lengths produce a narrower angle-of-view, thus leading to the subject looks bigger (or closer).
As long as the film/sensor size is unchanged, focal length and angle-of-view are inversely proportional (shorter focal length => wider viewing angle).
Due to the fact that sensor sizes vary significantly (from MFT to APS-C to Full-Frame and Medium Format), the same angle-of-view is produced using significantly different focal lengths.
As many lenses can be (and are even designed to be) used on several sensor/film sizes, it would be more sensible to refer to a lens’ angle-of-view (per sensor), but describing lenses by focal length has become an ingrained habit.
The angle of view corresponding to a normal lens (≈ 46°) is achieved by
- 25 mm focal length on MFT
- 31-33 mm focal length on APS-C
- 50 mm focal length on Full-frame
- 64 mm focal length on digital medium format (e.g. Fuji G)
- 75-92 mm focal length on classic medium format (i.e. 6 x 4,5/6/7 cm film)