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Focal length and angle of view

You probably know that a lens like a 28mm gives a relatively wide-angle view while, say, a 200mm lens gives a much narrower view, often suitable for distant subjects. So far so good. But as soon as you try and unpick what the numbers really mean, it gets confusing. Conventional ways of describing lens focal length seem designed to cause confusion.

The lenses of SLR cameras are designated by their actual focal length. Compact cameras have much smaller lenses and their true focal lengths are correspondingly smaller, but they are nearly always described by their ‘equivalent’ focal length. This is the lens that would give the same picture coverage on a 35mm camera or full-frame DSLR. Sometimes they’ll say ‘35mm equivalent’ – so you can even get a lens which is ‘35mm (35mm equivalent).’

Where it gets really confusing is with other SLR formats, such as the widely-used APS-C. This is just under half the size of 35mm/full-frame and therefore, naturally, gives less picture coverage, or angle of view, from any given lens. The same lens on a Nikon D700 (full-frame) and D7000 (APS-C) will give different results; an 18mm lens on the APS-size camera gives coverage equivalent to 27mm on full-frame (see photos). 18mm is commonly found as the wide end of DSLR kit lenses. On an APS-C camera this is 27mm-equivalent, so it gives a slightly less wide view than the 24mm-equivalent found on a few compacts – but on the SLR you can always fit a wider lens. On the compact you’re stuck with what you’ve got.