Patent ID: 6903804

Claim:
A projection optical system comprising, in sequential order from the object side: a first lens group having positive refractive power; a second lens group; a third lens group that has one of positive or negative refractive power; a stop; a fourth lens group that has the same sign of refractive power as the third lens group; a fifth lens group having the same sign of refractive power as the refractive power of the second lens group, the amount of refractive power of the second and fifth lens groups being small in comparison to the amount of refractive power of the third lens group; and a sixth lens group having positive refractive power; wherein the first and sixth lens groups are positioned at substantially symmetric locations on opposite sides of the stop; the second and fifth lens groups may be moved asymmetrically with respect to the stop along the optical axis from substantially symmetric locations on opposite sides of the stop so as to achieve magnification adjustment, and the third and fourth lens groups are positioned at substantially symmetric locations on opposite sides of the stop.