A modified optical system for the light microscope has been devised in order to remotely shift the focal plane and to manipulate the point spread function for any given objective lens. An adjustable telescope system is inserted into the microscope tube so as to move the intermediate image position, thus achieving two goals of fundamental importance for the three-dimensional imaging of biological samples. First, it allows the focus to be rapidly varied without actually moving the objective lens. This permits high throughput three-dimensional microscopy of living specimens. Secondly, it makes possible the compensation of objective lens spherical aberration. This distortion is especially significant when high numerical aperture objectives are utilized to image deep into thick specimens.