1. An apparatus for tracking the position of a molecule or particle in a sample, comprising: a stage for supporting the sample, a light source for emitting a light beam capable of inducing a measurable light-based response from a chosen molecule or particle in the sample, an objective lens for focusing the light beam on the sample to create an excitation volume, said objective lens projecting at least a portion of the light based response from the excitation volume to at least one image plane, a dichroic element for directing the light beam from the light source to the objective lens, at least one spatial filter at the at least one image plane for selecting a portion of the light based response and defining an optical probe volume for that portion of the light based response, means for introducing a controlled relative motion between the optical probe volume and the molecule or particle, and means for monitoring the light based response from the chosen molecule or particle, wherein said means for monitoring the light based response uses the light based response to control an overlap between the optical robe volume with the particle or molecule being tracked.