  We have systematically investigated substrate-strain effects on the
electronic structures of two representative Sr-iridates, a correlated-insulator
Sr$_2$IrO$_4$ and a metal SrIrO$_3$. Optical conductivities obtained by the
\emph{ab initio} electronic structure calculations reveal that the tensile
strain shifts the optical peak positions to higher energy side with altered
intensities, suggesting the enhancement of the electronic correlation and
spin-orbit coupling (SOC) strength in Sr-iridates. The response of the
electronic structure upon tensile strain is found to be highly correlated with
the direction of magnetic moment, the octahedral connectivity, and the SOC
strength, which cooperatively determine the robustness of $J_{eff}$=1/2 ground
states. Optical responses are analyzed also with microscopic model calculation
and compared with corresponding experiments. In the case of SrIrO$_3$, the
evolution of the electronic structure near the Fermi level shows high
tunability of hole bands, as suggested by previous experiments.
