  With recent developments in microscopy, such as stimulated emission depletion
(STED) microscopy, far-field imaging at resolutions better than the diffraction
limit is now a commercially available technique. Here, we show that, in the
special case of a diffusive regime, the noise-limited resolution of STED
imaging is independent of the saturation intensity of the fluorescent label.
Thermal motion limits the signal integration time, which, for a given
excited-state lifetime, limits the total number of photons available for
detection.
