lens 4 is moved toward the lens 3 so that doublet symmetry conditions are no longer satisfied with reference to fig5 the image blur is graphed as a function of the position z 2 of the lens 4 for s 1 300 mm and r 1 r 2 40 mm values of z 2 in the range 359 360 mm correspond to the location of the lens 4 satisfying double symmetric magnetic conditions the image blur has a minimum value of about 0 108 u03bcm for 356 mm u2266 z 2 u2266 357 mm this range can be expressed as a displacement u03b4z 2 along the z axis 10 from an initial position in which the doublet symmetry conditions are satisfied the displacement u03b4z 2 of the lens 4 toward the lens 3 is given by equ10 wherein the doublet symmetry conditions are satisfied if the displacement u03b4z 2 0 the blurs of fig2 5 are calculated for a fixed propagation angle u03b1 and coordinate u03b2 at the sensitized substrate 2 the blurs result mainly from third order geometric aberrations fig6 7 display image blur as a function of the propagation angle u03b1 and the coordinate u03b2 respectively in fig6 the image blur for the coordinate u03b2 0 5 mm is graphed as a function of the propagation angle.