A system, comprising: a first light source producing a probe light beam to be injected into an eye; a pre-correction system which compensates the probe light beam to be injected into the eye for aberrations in the eye, the pre-correction system being positioned in an optical path between the first light source and the eye, the pre-correction system including a variable focal length lens; a processor configured to adjust a focal length of the variable focal length lens; a switchable calibration light source disposed at a known position with respect to the variable focal length lens and configured to provide a calibration light beam to the variable focal length lens; and a wavefront sensor configured to receive light returned from a retina of the eye in response to the probe light beam, and in response thereto to output first image data, and to receive the calibration light from the variable focal length lens when the switchable calibration light source is on, and in response thereto to output second image data, wherein the processor is configured to receive the first image data, wherein the processor is further configured to adjust the focal length of the variable focal length lens to have an adjusted focal length so as to provide a desired characteristic to at least one of: the probe light beam injected into the eye, and the light received by the wavefront sensor from the retina, wherein the processor is further configured to switch on the switchable calibration light source while the variable focal length lens has the adjusted focal length, and to ascertain the adjusted focal length of the variable focal length lens from the second image data output from the wavefront sensor in response to the calibration light received from switchable calibration light source, and wherein the processor is further configured to determine an aberration of the eye from the first image data produced while the variable focal length lens has the adjusted focal length, and from the adjusted focal length ascertained from the second image data. a first light source producing a probe light beam to be injected into an eye; a pre-correction system which compensates the probe light beam to be injected into the eye for aberrations in the eye, the pre-correction system being positioned in an optical path between the first light source and the eye, the pre-correction system including a variable focal length lens; a processor configured to adjust a focal length of the variable focal length lens; a switchable calibration light source disposed at a known position with respect to the variable focal length lens and configured to provide a calibration light beam to the variable focal length lens; and a wavefront sensor configured to receive light returned from a retina of the eye in response to the probe light beam, and in response thereto to output first image data, and to receive the calibration light from the variable focal length lens when the switchable calibration light source is on, and in response thereto to output second image data, wherein the processor is configured to receive the first image data, wherein the processor is further configured to adjust the focal length of the variable focal length lens to have an adjusted focal length so as to provide a desired characteristic to at least one of: the probe light beam injected into the eye, and the light received by the wavefront sensor from the retina, wherein the processor is further configured to switch on the switchable calibration light source while the variable focal length lens has the adjusted focal length, and to ascertain the adjusted focal length of the variable focal length lens from the second image data output from the wavefront sensor in response to the calibration light received from switchable calibration light source, and wherein the processor is further configured to determine an aberration of the eye from the first image data produced while the variable focal length lens has the adjusted focal length, and from the adjusted focal length ascertained from the second image data.