A focusing apparatus for use with an optical system having a high NA objective lens includes an image forming and capturing mechanism for forming an image in an intermediate image zone and for capturing an image by receiving and refocusing light from a selected focal plane within the intermediate image zone, and a focus adjusting mechanism for adjusting the position of the selected focal plane within the intermediate image zone. The image forming and capturing mechanism includes at least one high NA lens. In use, spherical aberration introduced by the high NA objective lens is reduced.