Until now I thought that focal length is just another way of measuring how much a lens zooms: The higher the number the more it magnifies, the smaller the number the wider the angle and thus the less it magnifies. The Canon 70-200 f2.8 and the Tamron 70-200 f2.8 VC USM both have the same focal length, but the canon zooms more. Reviews say that the Tamron is equivalent to a 170mm Canon. So obviously zoom and focal length can't be exactly the same. So are they two different things or is Tamron selling a 170mm as a 200mm lens?
Answer
There seem to two things here. First is your use of the word "zoom". Most people use the word zoom to mean a lens that can change focal length. As opposed to a prime lens which has a fixed focal length. So a 300mm prime lens is not a zoom lens even though it is a fairly long (or telephoto) lens. Both the lenses you mention are zoom lenses as they can change focal length between 70mm and 200mm
The second point is about the Tamron lens being more like 160mm. The important words in that review are "at minimum focus". Most of the time the Tamron will be a (close to) 200mm lens. However when you try to focus it on something very close to the camera it's focal length will change, getting down to around 160mm at the minimum focus distance. It is a reasonably common thing in modern lenses zoom lenses - the review goes on to say that the Nikon 70-200 is actually more like 135mm when focused at its minimum distance. (Note that that review doesn't compare the Tamron to a Canon 70-200 so we don't know whether the Canon lens also exhibits focus breathing or not.)
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