It is shown for the first time we believe, that when a spherical wave
illuminates a certain type of diffracting screen, in addition to the e
xpected focal-shift effect, depending on the value of the Fresnel numb
er of the focusing system, a focal switch effect can appear, i.e., an
increase in the height of the lateral lobe of the axial-intensity dist
ribution over that of the central lobe. (C) 1996 Optical Society of Am
erica