A generalized method is proposed for computing the off-axes extinction ratio of Solc birefringent filters. Based on the result, the conoscopic figures of Solc birefringent filters are simulated, and the curves of integral extinction versus wavelength and incident angular aperture are presented in the case of diffusive incidence. The calculated values agree well with the experimentally obtained values given here. Moreover, the method is suitable for computing the off-axes extinction ratio of all kinds of birefringent filters.