An optical measurement system: passes a probe light beam through a variable focal length lens to the retina of an eye, and returns light from the retina through the variable focal length lens to a wavefront sensor； adjusts the focal length of the variable focal length lens to provide a desired characteristic to at least one of: the probe light beam, and the light returned by the retina to the wavefront sensor； passes a calibration light through the variable focal length lens to the wavefront sensor while the variable focal length lens is at the adjusted focal length to ascertain the adjusted focal length； and makes a wavefront measurement of the eye from the light returned from the retina of the eye through the variable focal length lens to the wavefront sensor, and from the adjusted focal length ascertained from the calibration light received by the wavefront sensor.
