Modern enthusiast-level compact cameras, i.e. those whose image sensor is smaller than “full frame” (24×36 mm or “35 mm”) and which may use fixed rather than interchangeable lenses, tend to be advertised with surprisingly excellent lens parameters. Generous zoom lengths of hundreds of millimeters are combined with very bright f-numbers of f/2 or less. Users of bulkier and more expensive full-frame cameras would pay a small fortune for such lenses. If you’ve noticed this discrepancy while shopping for a new camera you probably thought there must be a catch, and sure enough there is.
The fundamental issue is that lens parameters are relative to sensor size, and you must compare equivalent parameters between cameras of different sensor sizes – which is something manufacturers only partly do, to their own advantage of course. Smaller sensors make short lenses capture a smaller angle of a given scene, making them equivalent to long lenses on bigger sensors; but smaller sensors also cause bright lenses to capture less light from the same scene, making them equivalent to dark lenses on bigger sensors. This concept of equivalence is clearly and thoroughly explained by Richard Butler’s article at Digital Photography Review. I recommend you read these four pages yourself, but below I’ll summarize the pertinent points for camera buyers.
- Both focal length and f-number (indicating maximum aperture) of a lens must be considered relative to its camera’s sensor size.
- Using “full frame” (24×36 mm or “35 mm”) as a standard, you must multiply the focal length and f-number for smaller sensors by the ratio of diagonals. For example, a “four thirds” sensor has about half the diagonal of a full-frame sensor, so both values on the four-thirds lens would be doubled for equivalence with a full-frame lens.
- Compact camera makers always multiply focal lengths, giving them in “35 mm equivalent,” because greater zoom ranges look more impressive. So a 100 mm zoom lens on a four-thirds camera becomes a “200 mm” lens, as both capture the same angle of a given scene.
- Compact camera makers never multiply f-numbers, giving only the unadjusted values for the smaller sensor size, because lower f-numbers look more impressive. So an “f/2” lens on a four-thirds camera really only captures as much light as an f/4 lens on a full-frame camera.
Compact cameras also have the disadvantage that smaller sensors must either feature fewer pixels or cram pixels closer together than larger sensors at an equivalent technological level. In either case, image quality suffers – whether in resolution or in low-light performance. This, however, is relatively well known. The deceptive reporting of lens parameters is not, and frankly qualifies as advertising fraud in my opinion. You need to be aware of the need to mentally adjust f-numbers just like focal lengths.