Possible Duplicate:
How do I convert lens mm to optical zoom times?

How can I obtain the "X factor" for a lens?

I thought that to get the "X factor" I had to divide the largest focal length by the smallest one: using this my 18-55mm lens is a 3x one, and the 100-300mm would also be a 3x one, even if they are obviously very different lenses.



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