A method for controlling focus of an optical system. The method includes providing a pair of incident light beams to a conjugate lens. The incident light beams are directed by the lens to converge toward a focal region. The method also includes reflecting the incident light beams with an object positioned proximate to the focal region. The reflected light beams return to and propagate through the lens. The method also includes detecting a phase of each of the reflected light beams and determining a degree-of-focus of the optical system with respect to the object by comparing the phases of the reflected light beams.