  We report on zone center THz excitations of non-Jahn Teller LaFeO$_3$,
PrFeO$_3$, ErFeO$_3$, and LuFeO$_3$ distorted perovskites under external
magnetic fields up 7 T. Low temperature-low energy absorptions of LaFeO$_3$
show antiferromagnetic and ferromagnetic quasimagnons at $\omega$AFM ~31.4 and
$\omega$FM ~26.7 cm$^{-1}$ in the $\Gamma$4 (Gx, Ay, Fz) representation.
LuFeO$_3$ is characterized by zero field magnetic resonances at $\omega$AFM
~26.3 cm$^{-1}$ and $\omega$FM ~22.4 cm$^{-1}$ in addition to Fe$^{3+}$
Zeeman-split crystal field (CF) 6A$_1$ ground transitions at ~10.4 cm$^{-1}$
triggered by structural deviations induced by smaller Lu 4f$^{14}$. This local
non-centrosymmetric departure is also found in ErFeO$_3$ (Kramers 4f$^{11}$
Er$^{3+}$ (4I15/2); {\Gamma}2 (Fx, Cy, Gz) <TSR ~93 K), but with the ~4
cm$^{-1}$ Fe$^{3+}$ Zeeman branching strongly biased toward higher energies.
Quasimagnons at $\omega$AFM ~31.5 cm$^{-1}$ and $\omega$FM ~21.5 cm$^{-1}$ in
ErFeO$_3$ do not undergo field induced band splits but a 13-fold increase in
the antiferro ($\omega$AMF) /ferro($\omega$AFM) intensity ratio. There is a
remarkable field-dependent CF matching population balance between Fe$^{3+}$
higher and Er$^{3+}$ lower Zeeman branches. Antiferro- and ferro- resonances in
PrFeO$_3$ turn much broader as non-Kramers Pr$^3$ introduces ligand changes at
the A site leading into near degeneracy the antiferromagnetic mode and the
lowest Pr$^{3+}$ CF transition. We conclude that low energy excitations in
RFeO$_3$ (R=rare earth) strongly depend on the lanthanide ionic size. Minute
lattice displacements also underlie considering non-centrosymmetric the most
distorted RFeO$_3$ (R=rare earth). Changes triggered by the smaller rare earth
and the nonlinear intrinsic oxygen ion polarizability provide grounds for
interplay of ionic and electronic interactions yielding ferroelectric
spontaneous polarization.
