A wide-field interferometric microscope comprising: - a specimen holder, for holding a specimen at an analysis location; - an illuminator, for illuminating the specimen with input radiation, so as to cause it to emit fluorescence light; - a pair of projection systems (P1, P2), arranged at opposite sides of said analysis location, to collect at least a portion of said fluorescence light and direct a corresponding pair of light beams into a respective pair of inputs of an optical combining element, where they optically interfere; - a detector arrangement, for examining output light from said combining element, wherein - the illuminator comprises an optical cavity that is configured to produce a standing wave of input radiation at the analysis location, characterized in that - said optical combining element comprises a two-way beam-splitter, - the detector arrangement comprises exactly two interferometric detection branches, comprising a first and a second camera (Da, Db) for measuring intensity values, and wherein said illuminator comprises an adjustable optical retarding element or an adjustable mirror (7a, 13) allowing the phase of the standing wave to be adjusted, and wherein the microscope is configured to: taking a first intensity value measurement using the first camera and a second intensity value measurement using the second camera, altering the phase of the standing wave using the adjustable retarding element or the adjustable mirror, and taking a third intensity value measurement using the first camera and a fourth intensity value measurement using the second camera, the third and the fourth intensity value measurement being taken at the altered phase, using the first, second, third and fourth intensity value measurement to derive an axial position of an observed portion of the specimen. - a specimen holder, for holding a specimen at an analysis location; - an illuminator, for illuminating the specimen with input radiation, so as to cause it to emit fluorescence light; - a pair of projection systems (P1, P2), arranged at opposite sides of said analysis location, to collect at least a portion of said fluorescence light and direct a corresponding pair of light beams into a respective pair of inputs of an optical combining element, where they optically interfere; - a detector arrangement, for examining output light from said combining element, wherein - the illuminator comprises an optical cavity that is configured to produce a standing wave of input radiation at the analysis location, characterized in that - said optical combining element comprises a two-way beam-splitter, - the detector arrangement comprises exactly two interferometric detection branches, comprising a first and a second camera (Da, Db) for measuring intensity values, and wherein said illuminator comprises an adjustable optical retarding element or an adjustable mirror (7a, 13) allowing the phase of the standing wave to be adjusted, and taking a first intensity value measurement using the first camera and a second intensity value measurement using the second camera, altering the phase of the standing wave using the adjustable retarding element or the adjustable mirror, and taking a third intensity value measurement using the first camera and a fourth intensity value measurement using the second camera, the third and the fourth intensity value measurement being taken at the altered phase, using the first, second, third and fourth intensity value measurement to derive an axial position of an observed portion of the specimen.