Actual magnification of a camera lens

A 75-300 lens is said to have a 4X zoom range, not a 4x magnification.
The 4X zoom range actually means that 300mm is 4X greater than 75mm (75x4=300).

A 30mm to 300mm lens would have a 10X zoom range, but it would not bring anything closer than the 75 to 300mm lens (with a 4X zoom range), since the maximum focal length in both is the same.

It is generally accepted that for a 35mm film camera, a 50mm lens most closely approximates what our eyes see (in reality it should be 42mm - always using the 35mm format).
By dividing the focal length of your lens by 50, you can estimate the actual magnification.

A 300mm lens (on a 35mm film camera or a full frame digital camera) would have a magnification factor of 7X (300/42=7.1).
When used on a Nikon D90 (smaller sensor than a full frame camera - 1.5 crop factor) then you would have a magnification of 11X (300/28=10.7). You could say that a 300mm lens on a Nikon D90 is roughly the same as using a 11X telescope..

A 1200mm scope will have a zoom factor of 1X and a magnification of 29X (1200/42=28.5).
And on a Nikon D90 it will have a magnification of 43X (1200/28=42.8).

On a Nikon 1 system (Nikon 1 J1 and V1 models 2.7 crop factor) it will have a magnification of 77X (1200/15.55=77.1 or 3240/42=77.1).

A Nikkor 400mm f/2.8 lens when used on a Nikon V1 with a FT-1 adapter + TC 2.0X (400*2*2.7=2160mm focal length) will have a magnification of 51.4X!!! (http://nikonrumors.com/2012/01/17/some-nikon-v1-ft-1-tc-2-0x-400mm-f2-8-fun-2160mm.aspx/)

A Nikkor 2000mm lens will have a magnification of 48X (35mm format) and when used on a Nikon 1 system it will have a magnification of 128.6X!!!!!!!

A 5200mm Canon lens will have a magnification of 334.3X!!!!!!!!!!! if used on a Nikon 1 system (with an adaptor..).

In 1609 Galileo made a telescope with about 3X magnification. He later made improved versions with up to about 30X magnification!