Consumer photographic lenses are almost always sold by their focal length when used on a 35mm camera (or full-frame professional DSLR). It’s already confusing enough to work out what their effective focal length is when used on an APS-C sensor (most consumer DSLRs), but since I’ve been getting into medium-format photography it’s even more confusing to work out what each focal length does what on which camera.
So I’ve done a little research, and prepared a table. I’ve included data for five “common” photography formats with their crop factors. The numbers are the approximate focal lengths in millimetres. Of course the boundaries for each category of lens are rather woolly at best.
| Film or sensor format | |||||
|---|---|---|---|---|---|
| Lens type | APS-C | 35mm | 6×6cm | 6×7cm | 5×4in |
| Fish eye | 10 | 16 | 30 | 35 | 45 |
| Super wide | 12 | 20 | 40 | 45 | 65 |
| Wide angle | 17 | 28 | 50 | 55 | 90 |
| Normal | 30 | 50 | 80 | 90 | 150 |
| Short telephoto | 49 | 80 | 150 | 180 | 270 |
| Medium telephoto | 123 | 200 | 350 | 420 | 520 |
| Long telephoto | 185 | 300 | 500 | 600 | 800 |
Jonathan Gazeley 
A focal length is surely a property of the lens. Any mangling of the number is incorrect.
I have two bodies I use in my day-to-day work, one full-frame and one APS-C. I guess I have an instinct as to what each lens will do on each camera because I sure as heck haven’t bothered doing any tests! (Though I think I do a mental 1.5x* up and down convert sometimes… like if I need wide-angle, I’ll think… is 17mm on APS-C wider than 24mm on full frame? Even at 1.5x is isn’t, so I’ll go with the 24mm).
(*1.5 is close enough for rock’n’roll!)
And before you suggest it, no – the 17mm won’t go on the full-frame because it’s an EF-S lens.
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Interesting thread here:
http://digital-photography-school.com/forum/nikon-digital-cameras/150601-dx-does-not-get-same-results-fx.html
In short, the focal length of a lens intended for use with APS-C sensors (e.g. Nikkor DX) is the same as that of a lens intended for full-frame 35mm sensors/film (e.g. Nikkor AF, AF-G or AF-D). However, an APS-C-targeted lens will give a heavily vignetted image on a FF body.
An APS-C body will only record a crop of the image projected, meaning that if you want to achieve the same field of view with a FF body, you’ll also need a lens with a focal length 1.5x (Nikon) or 1.6x (Canon) of whatever you were using on the APS-C body.
Finally, from the thread linked above, the focal length you actually get depends on how far the subject is from the lens!
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