I've read Chuck Anstey's rather technical article on sub exposure length (and number), but I'm unable to calculate my own optimal exposure length, as I don't have the tools to normalize pictures… And I'd likely make too many mistakes along the way.
According to light pollution map, I'm in a Bortle class 5 zone with a sky brightness of 19.9 mag/Arcsec².
I'm using a stock Canon 6D through an old f/7 triplet, shooting at ISO 800.
Any good suggestions on sub length?
I actually made a test, shooting 2x20min at M45, one consisting of 32x30sec., the other 8x120sec.
Temperature was about 10°C/50°F. All stacked in deepskystacker, and the resulting stack was considerably more bright with the long exposures - Makes sense.
I ran the same actions on them in PS, doing 9 consecutive curves raises. After that, I set blackpoint in levels at approximately the same spot (5x5 sample).
The results seemed to indicate that 8x120seconds was a little better than the shorter exposures. The stars are generally smaller, and the longer subs have surprisingly more blue in the nebulosity/dust… I've tried redoing the blackpoint several times on the shor-sub picture, no change. But I might be completely off on the wrong track here, or missing something obvious.
All comments or suggestions appreciated. Next time the moon&clouds allow, I might try 180sec. Vs. 60sec instead…