In a particle-optical projection system a pattern is imaged onto a target by means of energetic electrically charged particles. The pattern is represented in a patterned beam of said charged particles emerging from the object plane through at least one cross-over; it is imaged into an image with a given size and distortion. To compensate for the Z-deviation of the image position from the actual positioning of the target (Z denotes an axial coordinate substantially parallel to the optical axis), without changing the size of the image, the system includes a position detector for measuring the Z-position of several locations of the target, and a controller for calculating modifications of selected lens parameters of the final particle-optical lens and controlling said lens parameters according to said modifications.