1. A microscope comprising: an illumination system configured to illuminate at least one particle; and an optical system configured to image light from the at least one particle, the optical system comprising: an imager; an optical element configured to modulate light from the illumination system so that the light includes two intensity spots separated by a first distance along a first axis and separated by a second distance along a second axis, wherein the second distance depends on the axial position of the particle, and wherein the first axis and the second axis are orthogonal; and one or more lenses configured to direct light from the illumination system and the optical element to the at least one particle, and direct light from the at least one particle to the imager.