PROPAGATION OF FOCAL SHIFT THROUGH AXISYMMETRICAL OPTICAL-SYSTEMS

Focusing of an un-apertured spherical gaussian beam is re-examined on the basis of transformation of the focal shift, namely, transformation of the deviation of focal spot from its geometrical optics position to the plane containing the waist of gaussian beam. It is found that the relative focal shifts for gaussian beams in the object and image spaces of an axisymmetric optical system are coupled by the same geometrical optics relationship concerned with longitudinal magnification of small objects on the axis. However, the relative focal shifts in the object and image spaces are not necessarily small quantities, when the law of small object magnification is applied to describe the transformation of focal shift.