A method for performing differential interference contrast microscopy on a specimen includes collecting at least two images with illumination respectively having first and second beam-shear directions relative to a rotational orientation of the specimen, determining data associated with an intensity distribution of each of the collected images, and calculating values having a spatial distribution that is substantially independent of the rotational orientation of the specimen. A differential interference contrast microscope includes a beam-shearing assembly that includes a beam-shearing component. The beam-shearing assembly is configured to provide a variable shear vector without a movement of the beam-shearing component. A microscopy system can include the microscope and an imaging-control unit.