Narrowband filter sets are used to create high contrast deep sky images of certain objects, mainly emission and diffuse nebulae (i.e. Veil Nebula, M42 Orion Nebula, North America Nebula, Horsehead Nebula) or planetary nebulae
The ZWO OIII 7nm narrowband filter is designed for nebula observation allowing 7nm bandwidth of light centered on a wavelength of 500nm, which corresponds to OIII emission lines, blocking out all other light.
The ZWO SII 7nm narrowband filter passes light at 672nm wavelength with a bandpass of 7nm which is designed for nebula observation and imaging. It is suitable for visual observation on most emission nebulae