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Gday Hans - I'll have crack at it
People have their own preferences but as I understand it - 3 nm = narrower section of the spectrum = longer exposures on fainter objects but also = higher contrast. I have heard of people having focusing problems but I suspect that might be with DSLRs and focusing visually - oh yeah good quality = good dollars they cost more to get a good one See Astrodon 3nm for example - I used to pay that for a scope but again they are quality gear.
I use 6 nm Astronomik - double spectral range but still fairly high contrast - still a bit expensive to get a good one - i've never had any drama focusing - a 1 second exposure on a 4th mag star is fine with my camera and scope.
I have even used a 12 nm visual Ha filter - ok if you are starting out but might be a waste of time. Advantage - cheap. Nah stay away from em
I am sure others have more experience and technical knowledge but this is my experience.
Mark Bolton
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