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Lateral resolution describes distinguishable separation in a plane perpendicular to an imaging system's axis. For incoherent point objects imaged through a circular aperture, the Rayleigh criterion gives the specimen-space scale $d\simeq 0.61\lambda /\mathrm{NA}$, where $\lambda$ is the vacuum wavelength and $\mathrm{NA}$ the numerical aperture.^[ref]

For example, $\lambda =550\mathrm{nm}$ and $\mathrm{NA}=1.0$ give $d\simeq 336\mathrm{nm}$ under that criterion. Sampling the image with more pixels can represent this blur more accurately, but cannot by itself distinguish closer features. Comparisons between instruments must specify both the object and the criterion.

Optical resolution is the ability of an imaging system to distinguish spatially separated features under specified observation conditions. A finite numerical aperture spreads a point object into a point spread function; neighboring point images overlap. A criterion such as the Rayleigh criterion assigns a separation to this overlap rather than marking a sharp universal boundary between visible and invisible detail.^[ref]

A practical test images two small sources at known separations, keeping exposure and display image contrast fixed. Decreasing their separation until the chosen criterion fails measures lateral resolution; moving them along the imaging axis tests axial resolution. Magnification alone cannot narrow the specimen-space point spread function.