The Klein‐Gordon equation with the external electromagnetic field is unambiguously formulated by the two component theory as shown by a lucid presentation of Feshbach and Villars1. Following Hamiltonian formalism, they wrote down a definite form of the Klein‐Gordon equation with a real, non‐momentum dependent potential for the strongly interacting pion system nearly two decades ago. In this pedagogical note we describe the Klein‐Gordon equation with less restricted optical potentials, relying on our guidance in field theoretical lagrangian.