  We present a comparison between lens cavity filters and atomic line filters,
discussing their relative merits for applications in quantum optics. We
describe the design, characterization and stabilization procedure of a lens
cavity filter, which consists of a high-reflection coated commercially
available plano-convex lens, and compare it to an ultra-narrow atomic band-pass
filter utilizing the D$_{2}$ absorption line in atomic rubidium vapor. We find
that the cavity filter peak transmission frequency and bandwidth can be chosen
arbitrarily but the transmission frequency is subject to thermal drift and the
cavity needs stabilization to better than a few mK, while the atomic filter is
intrinsically stable and tied to an atomic resonance frequency such that it can
be used in a non-laboratory environment.
