The excitation of the E- and H- modes by focusing of a monochromatic light beam onto a hollow metal capillary (MC) entrance is theoretically detailed. As found, a numerical aperture sets a stringent bound on the number of excited optical modes, either when the beam is focused by a spherical lens onto the entrance of a circular MC or when a rectangular capillary is illuminated via a cylindrical lens. Transmission characteristics calculated in a general form provide grounding to use MCs as a means to enhance the Raman signals.