Over at Cloudynights the same very faint target (Sh2-129 and OU-4) have been imaged very recently by Nicola (who also posts here) with a QSI583 CCD and by Scott Rosenfraz with a modded Canon 450D DSLR.
Nicola's CCD image:
http://www.cloudynights.com/ubbthrea...l/fpart/1/vc/1
Scott's DSLR image:
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
The scope that Nicola uses - Tak FSQ85ED - is nominally F/5.3, not sure if he used a focal reducer.
Scott used a 200mm F/2.8 lens - 2.5(?) F-stops faster, yet total exposure time is much, much great than Nicola's image.
Hard to make too many judgements. There should be more signal-to-noise in Scott's H-a data, Sqrt(99 subs)=10 x 10-minute subs. Nicola used sqrt(8 subs)=3 x 20-minute subs. I think they look almost identical. That ratio of normalised exposure would give the difference at 656nm in sensitivity. So by my very rough calculation we'd be seeing 1.67 (normalised exposure difference) x 2.5 (F-stop difference) = 4x difference at H-alpha.
Lots of caveats of course,
Cam