Editing Magnification (section)

==Maximum usable magnification==
With any telescope, microscope or lens,
a maximum magnification exists beyond which the image looks bigger but shows no more detail.  It occurs when the finest detail the instrument can resolve is magnified to match the finest detail the eye can see.  Magnification beyond this maximum is sometimes called "empty magnification".

For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by [[Diffraction-limited system|diffraction]].  In practice it is considered to be 2× the aperture in millimetres or 50× the aperture in inches; so, a {{val|60|u=mm}} diameter telescope has a maximum usable magnification of 120×.{{citation needed|date=October 2012}}

With an optical microscope having a high [[numerical aperture]] and using [[oil immersion]], the best possible resolution is {{val|200|u=nm}} corresponding to a magnification of around 1200×. Without oil immersion, the maximum usable magnification is around 800×.  For details, see [[Optical microscope#Limitations|limitations of optical microscopes]].

Small, cheap telescopes and microscopes are sometimes supplied with the eyepieces that give magnification far higher than is usable.

The maximum relative to the minimum magnification of an optical system is known as '''zoom ratio'''.