a projection optical system which sufficiently corrects variations in aberrations during focusing irrespective of a distance at which an object is located , i . e . whether the object is located at a point of infinity or at a short distance , while maintaining advantages of a rear focus retrofocus type optical system , is disclosed . this projection optical system has , in order from the enlarging side , a first lens unit having negative optical power , and a second lens unit having positive optical power and moves in a direction of an optical axis for focusing . the following condition is satisfied : 0 ≦| f / t 1 |& lt ; 0 . 2 where reference character t 1 denotes a distance from the reducing - side conjugate plane to a pupil and reference character f denotes a focal length of the entire system .