A 3D localization microscopy system, 4D localization microscopy system, or an emitter tracking system arranged to cause a phase difference between light passing to or from one part of the objective relative to light passing to or from another part of the objective, to produce a point emitter image which comprises two lobes, a separation between which is related to the position of the emitter relative to the objective of the imaging system, and in the 4D system a further property of the image or of the light to or from the objective is related to another location independent property of the emitter.