  The optical conductivities of films of
(La$_{0.5}$Pr$_{0.5}$)$_{0.7}$Ca$_{0.3}$MnO$_3$ with different oxygen isotopes
($^{16}$O and $^{18}$O) have been determined in the spectral range from 0.3 to
4.3 eV using a combination of transmission in the mid-infrared and ellipsometry
from the near-infrared to ultra-violet regions. We have found that the isotope
exchange strongly affects the optical response in the ferromagnetic phase in a
broad frequency range, in contrast to the almost isotope-independent optical
conductivity above $T_C$. The substitution by $^{18}$O strongly suppresses the
Drude response and a mid-infrared peak while enhancing the conductivity peak at
1.5 eV. A qualitative explanation can be given in terms of the phase separation
present in these materials. Moreover, the optical response is similar to the
one extracted from measurements in polished samples and other thin films, which
signals to the importance of internal strain.
