  We investigated the $ab$-plane absorption spectra of $R$MnO$%_{3}$ ($R$ = La,
Pr, Nd, Gd, and Tb) thin films. As the $R$-ion size decreases, we observed a
drastic suppression of the 2 eV peak, \textit{i.e.} the inter-site optical
transition between spin- and orbital-aligned states across the Mott gap. We
found that both lattice distortion and the corresponding orbital mixing of the
ordered orbital state should play an important role in the 2 eV peak
suppression. We also found that the 2 eV spectral weight is proportional to the
$A$-type antiferromagnetic ordering temperature, which suggests that the
magnetic interaction might be sensitively coupled to the orbital mixing.
