Narrowband filters pass only a sliver of spectrum (typically 3–7nm) around a single emission line — Hα (656nm), OIII (501nm), or SII (672nm). Because nebulae emit strongly at these lines while light pollution and moonlight are broadband, narrowband filters reject 99%+ of unwanted light, enabling deep emission-nebula imaging from bright cities and even under a full moon. The cost: only emission targets benefit (not galaxies/broadband), and per-channel exposures are long. Narrower filters reject more pollution but cost more and demand precise focus.