The effect on confocal imaging of spherical aberration caused by a weak ref
ractive-index mismatch is discussed. Aberration balancing that uses a chang
e in the objective's tube length is studied. It is found that the range of
depths that can be imaged satisfactorily by a high-numerical-aperture objec
tive with compensation is an order of magnitude greater than that without c
ompensation. The aberration balancing tends to break down for extremely hig
h numerical apertures. (C) 2000 Optical Society of America. OCIS codes: 080
.1010, 110.0180, 180.1790, 260.1960, 110.2990.