A method of determining the position of a probe tip. An evanescent electromagnetic field is generated extending beyond an interface boundary between a first medium, having a first refractive index, and a second medium, having a second refractive index which is greater than the first refractive index, the interface boundary extending in a plane. A probe tip is positioned in the evanescent field in the first medium thereby causing propagating electromagnetic radiation to be produced as a result of the disruption of the evanescent field by the probe tip, and at least a portion of the propagating electromagnetic radiation is collected. The spatial intensity distribution of the collected radiation is detected with respect to an image plane. An at least one dimensional position of the probe tip in a probe tip plane is determined from the detected spatial intensity distribution, the probe tip plane being a plane which contains the probe tip and which is substantially parallel to the plane of the interface boundary.