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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?