Microscope focus control system

A microscope focus control system comprising:

• an illumination source, and a polarization-sensitive detector positioned to receive a reflected beam from an interface having a reference refractive index and configured to detect polarization components of the reflected beam,
• wherein an elongate rectangular aperture is disposed proximal the illumination source to generate a rectangular beam to impinge on the interface,
• a converging lens is located in the path of the reflected beam to focus the reflected beam towards the polarization-sensitive detector, and
• a second aperture is located at a position corresponding to an unfocused region of the reflected beam from the interface, between the converging lens and the polarization-sensitive detector, so that the extent to which the rectangular beam is occluded by the second aperture changes as the focus of the reflected beam changes, such that resulting changes in polarization components of the reflected beam are used as a measure of change in position of the interface relative to an objective.