Let (X, 0) be a complex analytic surface germ embedded in $$({\mathbb {C}}^n,0)$$
(
C
n
,
0
)
with an isolated singularity and $$\Phi =(g,f):(X,0) \longrightarrow ({\mathbb {C}}^2,0)$$
Φ
=
(
g
,
f
)
:
(
X
,
0
)
⟶
(
C
2
,
0
)
be a finite morphism. We define a family of analytic invariants of the morphism $$\Phi $$
Φ
, called inner rates of $$\Phi $$
Φ
. By means of the inner rates we study the polar curve associated with the morphism $$\Phi $$
Φ
when fixing the topological data of the curve $$(gf)^{-1}(0)$$
(
g
f
)
-
1
(
0
)
and the surface germ (X, 0), allowing to address a problem called polar exploration. We also use the inner rates to study the geometry of the Milnor fibers of a non constant holomorphic function $$f:(X,0) \longrightarrow ({\mathbb {C}},0)$$
f
:
(
X
,
0
)
⟶
(
C
,
0
)
. The main result is a formula which involves the inner rates and the polar curve alongside topological invariants of the surface germ (X, 0) and of the curve $$(gf)^{-1}(0)$$
(
g
f
)
-
1
(
0
)
.