Optical measurement of focal offset in tunable lenses

Electrically tunable lenses are becoming a widely used optical tool, and have brought significant innovation to microscopy methods. One current limitation of such systems is the difficulty of directly monitor the focal change in real time. Affordable and reliable feedback for such lenses, compatible with any microscopy setup, represents a much-needed improvement that is still not widely available. We discuss here the implementation and technical performance of an optical device to measure with a high frequency response the displacement of the focal offset of a commercial tunable lens with a precision in the range of the axial Point Spread Function (PSF) of the microscope. The technology presented is cost effective and can be employed on any microscopy setup.