From my experience comparing Optolong UHC, Optolong L-Enhance (27nm?), Svbony Sky Glow:
Sky Glow - hard to see a difference between filtered and unfiltered view under Bortle 6 skies, even harder under dark skies. But I like how it works on Jupiter.
Optolong UHC makes a decent improvement on nebulae. About 80% of that of L-Enhance under brighter skies or for low power faint objects. Under dark skies and 2mm exit pupil it's performance is pretty much equal to L-Enhance
As nebula filters usage moves toward the 2.5mm exit pupil range, the effectiveness of the filters drops.
The reason is that the background is already dark, so background darkening has less effect, and the nebula itself is getting dimmer.
It is for this reason that 10x/inch of aperture (a 2.5mm exit pupil) is usually recommended as the highest power to use a nebula filter with.
However, there is not really a ceiling for the filters. It depends on bandwidth.
A wider filter, like a broadband, can be used at a somewhat higher power (say, up to 12-13x/inch, or a 2mm exit pupil) without excessively dimming the field or nebula.
That would explain why you see little difference between the Optolong UHC (47-48nm bandwidth) and the L-Enhance (27nm bandwidth) at a 2mm exit pupil. The L-Enhance would
provide more contrast but with the darker background, its superiority to the UHC would be significantly reduced by the higher power of a 2mm exit pupil.
At lower powers (4mm exit pupil, maybe), the L-Enhance should work much better.
What you could also use is a decent dual-wavelength (495.9nm and 500.7nm) O-III filter for use on planetaries, supernova remnants, and Wolf-Rayet excitation nebulae.
Your L-Enhance should be a fine narrowband filter for the large hydrogen gas clouds (e.g.M8, M20, M17, M16, M42).
Look for something in the 11-16nm bandwidth for optimum enhancement, like:
Lumicon O-III Gen.3
Intercon Spacetec (ICS) O-III (made by Astronomik)
Tele Vue Optics Bandmate II O-III (made by Astronomik)
Astronomik O-III Visual
DGM Optics O-III
Edited by Starman1, 30 June 2022 - 09:10 AM.