Oxygen III line filters have a narrow passband centered on the 4959Å and 5007Å emission lines of doubly ionized oxygen where emission nebulas radiate strongly. The filters pass 85-95% of the light of these emission lines. At the same time, they reject almost all of the light from all other sources – city street lights, natural airglow, broadband starlight, etc. This increases the contrast of emission nebulas by making them stand out against a considerable dimmed sky background. They will dim much of the reflected starlight of a reflection nebula, however, and nebulas with a strong reflection component will often look better in a filter with a wider pass band.
Oxygen III filters generally have better than twice the contrast of a narrowband filter (four times the contrast or more compared with a broadband filter). They are good for observing very faint planetary and gaseous nebulas (such as M76 and Barnard’s Loop in Orion) that are normally invisible without a filter, even from relatively dark sky sites. They also give excellent results on brighter nebulas like M17 and M27 from light-polluted suburban locations. They are often your only hope for seeing faint nebulas from heavily light-polluted locations.
Oxygen III filters work best with medium to large aperture telescopes (6” to 8” and larger). They are for visual use only and cannot be used for photography.