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I've read How do I convert lens focal length (mm) to x-times optical zoom?, and learned that the times-zoom number doesn't tell you much. So, how do you determine the magnification — the amount a lens let's you "zoom in" — from focal length data? Why don't the lens manufacturers just tell you? Maybe there is another factor involved?
For example, 15-45mm and 100-300mm are both "3x zoom", but doesn't the 300 offer more "zoom in" capability or magnification than the 45mm lens?
If so then how does one measure the magnification abilities of a lens?