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The "zoom factor" is calculated like this:
So, the 70-300 is "only" a 4.3x zoom, but this tells you nothing about the actual reach of the lens.
The 300mm end of the 70-300 still gives you more "zoom" (reach) than the 270mm end of the 18-270.
Likewise, the Sigma 200-500mm has a zoom factor of 500/200=2.5x, but it's 500mm end is much longer, obviously, than the 70-300 (which is a 4.3x zoom).
It is likely best to abandon the term "zoom factor" altogether and learn about focal length.
Edited on Nov 21, 2008 at 03:44 PM · View previous versions