Extremely narrow filters like I mentioned (0.15nm) may result in "interesting" effects because of redshift/blueshift.
For example,
z = 0.0001 for Trapezium in Orion. This gives a shift (redshift) of Ha line by 0.06nm. This is a almost 1/2 of the FWHM of the 0.15nm filter, hence for that region the star intensity will be almost 50% dimmer than in a "wider" filter.
Faster moving objects may not even be visible through the filter...