Spatial resolution.
Common mistake, equating magnification of a lens with spatial resolution. The limit of resolution is indicated by the numerical aperture (number printed on the side of a lens, typically 0.1 to 1.49) and achieved by projecting the light onto a detector such that the limit is achieved.
Look at this webpage to see the difference between two 40X lenses. In the power spectrum picture at the bottom, one has a wider spread, a bigger circle. This is explained in Fig 1 "Fourier Space" of this paper http://jcb.rupress.org/content/190/2/165/tab-figures-data .
Figure 3 from the same paper has become a standard illustration of spatial resolution in different microscopy techniques. Although newer techniques since 2010 aren't listed such as commercial AiryScan and iSIM and non-commercial lattice light sheet.
last updated 20180606