For the theoretical study of Šolc filters, used in particular in solar observatories or coronagraphs, we emphasize the interest of the polarization representation using the complex plane. We show that, with some approximations allowed by the small angle of rotation between two successive filter plates, the traditional but highly computational study of basic Šolc filters is much shorter, and that a new path for the study of less basic Šolc filters is offered by the technique of calculation in the complex plane. This provides a stronger mathematical basis for previous research into tunable or apodising Šolc filters -- which is welcome as the relative height of the secondary maxima just near the transmission peaks of basic Šolc filters is 11%.