We present an overview of recent theories for describing specimen-indu
ced spherical aberration in confocal microscopy. One of these theories
is used to compute numerically the role of spherical aberration in ge
neral confocal, and especially in biological confocal, microscopy for
a variety of three-layer specimen structures. In particular, we study
the effect of specimen-induced spherical aberration on the maximum val
ue of the overall confocal point spread function, the accompanying foc
al shift and the size of the optical probe in both fluorescence and br
ightfield confocal microscopy.