1. An apparatus comprising: a position-sensitive detector configured for detecting radiation as a function of position on the detector; an optical system characterized by a diffraction-limited resolution volume and configured for imaging radiation emitted from optical labels in a sample onto the position sensitive-detector, wherein the optical labels are distributed in the sample with a density greater than an inverse of the diffraction-limited resolution volume of the optical system; a radiation source configured to provide radiation to the sample to induce emission of fluorescence radiation from a plurality of optical labels in the sample; a controller configured to: a) control the radiation source to induce emission of fluorescence radiation from a sparse subset of the plurality of optical labels in the sample such that a mean volume per fluorescing optical label in the subset is less than the diffraction-limited resolution volume of the optical system; b) control the position-sensitive detector to record an intermediate image of the subset of the optical labels; and c) repeat a) and b) wherein the radiation source is controlled to induce emission of fluorescence radiation from a different sparse subset of the plurality of optical labels.