Patent ID: 7161741

Claim:
An optical system with a variable focal state for viewing and measuring an object, said optical system having an optical axis, a focal length, an entrance pupil, and a first principal plane, said entrance pupil and said first principal plane having positions along said optical axis, comprising: (a) at least one optical element for forming an image of said object; (b) an image viewing plane for viewing said image of said object; (c) a component moveably disposed along said optical axis, said component having a range of positions along said optical axis, the position of said component determining said focal state of said optical system; (d) said optical system being so adapted that said entrance pupil remains at a substantially fixed position along said optical axis as said focal state is varied; (e) said optical system being further adapted so that said positions of said entrance pupil and said first principal plane are spaced apart by a distance which is substantially different than said focal length for at least one position of said component; and (f) means for measuring the position of said component along said optical axis.