The narrowband filters do make a HUGE difference to the visibility of nebulae even at a very dark site. Here is a comparison (sort of, since integration times were not equal) using a not particularly narrowband filter (40nm passband at Ha). Worth a click, IMHO. Generally I found that you need subs about 3x as long as for white light, minimal length for white light at a dark site is one minute at f2.8, narrowband 3-minutes. Adjust subexposure length for focal ratio. Since the subs can get very long for slow OTAs one might wish to start with a very short lens. A Canon 50mm/1.8 or 50/1.4 are excellent at f2.8 in Ha, and almost any mount should be able to track that for 3 minutes unguided. Note: because there is NIL chromatic aberration when shooting narrowband, f2.8 works well with these 50mm lenses, but in white light one needs to close down to f4. I have also come to the conclusion that the bulk of the imaging time (say, 75+%) should be with the narrowband filter, the white light is really just to provide star colors.