Patent ID: 8498048

Claim:
A focusing apparatus for use with an optical system having a high numerical aperture (NA) objective lens, the apparatus including: an image forming and capturing means configured to receive light from the high NA objective lens, wherein the image forming and capturing means includes a high NA lens that focuses the light received from the high NA objective lens to form an image in an intermediate image zone, and a reflective element that reflects the image formed by the high NA lens back into the same high NA lens to capture an image from a selected focal plane within said intermediate image zone; a focus adjusting means that adjusts an axial position of the reflective element to adjust a position of the selected focal plane within the intermediate image zone, whereby spherical aberration is reduced; a polarizing beam splitter that divides reflected light from the light entering the focusing apparatus; and a polarization adjuster located between the polarizing beam splitter and the intermediate image zone for adjusting polarization of the reflected light.