Super-resolution images of: (a) actin filaments stained with phalloidin-Alexa fluor 647 on glass. In (a) the Precision Limit of resolution is calculated as 53 nm and the limited sampling resolution is 80 nm, using Equations 1 and 8. The FWHM of the actin filament in the grey boxed area is plotted as an inset histogram to show the empirical resolution, and these FWHM values range from 56 nm to about 110 nm, with a mean of 79 nm. The empirical resolution is broadly consistent with the blind-assessed values that were calculated. The spread of FWHM arises from variable localisation precision, and from random permutations of the finite number of localisations that comprise each cross section. The localisation density of a cross section through two crossing filaments (red line) shows that features can be resolved at separations larger than the calculated resolution limit, as expected. (b) Epidermal growth factor (EGF) is conjugated to Alexa fluor 647 on HeLa cell surfaces, and bound to EGF receptors. Activation of EGF receptors results in dimerisation of receptors and clustering into pits and vesicles with diameters ranging from 50–150 nm. A Precision Limit of 60 nm was calculated from the raw data, which provides the user with confidence that the sub-diffraction sized clustering is a real structure.