Actually 3X, 4X etc is a relative term.
It is used mostly in point & shoot cameras which is meant for 'all-purpose'.
The zoom for the point and shoot camera is considered as max telephoto/max wide angle.
Most of the p&s camera max wide angle starts at 28mm. So a 10X p&s means 28-280mm.(280/28 = 10x).
Now for an SLR lenses, term nX is not used. Coz not every lenses are general purpose lenses.
Assume there is a lens 10-40mm is 4X
similarly a 100-400mm is also 4X.
These lenses will be used for different purposes. First one is a super wide angle lens and will be used mostly for landscape photography. Second one is a super telephoto lens and will be used mostly for bird/insect photography.
In a nutshell the X notation is a relative term where as A-Bmm is an absolute term.
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