What is the maximum magnification power a telescope or spotting scope can achieve?

As described above, the higher the magnification level, the dimmer an image becomes. There is thus a golden rule for the maximum usable magnification (Mmax) of any scope: Mmax=2 x D (mm)

Where: D is objective lens diameter measured in mm.

For example, for the Nipon 26-78x78 compact zoom scope, its objective lens is 78mm, so the maximum usable magnification power of this scope is 2x78=156 times.