In practice: ‘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 standard 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)
Technically, however: Focal length is the measure from the middle of a lens (single lens element or group of lens elements) to a specific point at which rays that have entered the lens in parallel will meet. Problematically the whole concept of focal length (in the technical sense) becomes somewhat unintuitive when considering multi-element lenses, because in these the concept of the ‘middle of the lens’ becomes less straightforward…
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