In order to study the validity of general focal length function in designing diffractive microlenses with long focal
depth, diffractive microlenses with different f-numbers are designed using general focal length function and their
focusing characteristics, such as real focal depth, real focal spot size, and diffractive efficiency, are investigated
using electromagnetic theory and boundary element method. Investigation results indicate that general focal
length function can be used to achieve long focal depth in designing diffractive microlenses, even twice over than
those of conventional diffractive microlenses with similar parameters.