  In this paper, focal shift of a Guassian laser beam tightly focused through
several planar dielectric interfaces is discussed. It is shown that the focal
shift and focus point can be changed by variation of refractive index mismatch
and this shift could be positive or negative, and it is proportional to
refractive index mismatch between coverglass and immersion medium. It is shown
also that for every immersion medium(coverglass), focal shift could be removed
for a continuous range of depths by choosing a coverglass (immersion medium)
with appropriate refractive index.
