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Hi everyone,
I've got the following approach:
I did some test a year ago with my D70 to determine the S/N ratio for different ISO settings, and found that any setting over 320 ISO gives the same S/N ratio, but the lower ISO settings give more banding and streaking on dark backgrounds. This is why I use the lowest ISO setting without getting background banding, in praxis this means the lower the f-ratio and the higher the background brightness (light pollution or Milky way), the lower the ISO setting can be.
Under a dark sky and an f-ratio of around 7, I usually end up with 800 ISO.
If for a given setting the brightest part of the object is going to be overexposed, I'll make a few extra short exposures to fill the brightest parts in.
For a given total exposure time shorter exposures will increase the dynamical range and dampen the effects of any stuff-up, but will add extra read-out noise, memory and processing time.
It will depend on the dynamical range of the object, the used hardware and the stability of tracking what determines the best time for the individual exposures, im my case a 10 minute duration works the best.
Consequently my master dark is shot with the same individual exposure duration and ISO setting, thanks to dark frame synthesizing I can use it for different ISO settings and exposure times as well (like my Milky way shot for 30 minutes @ 400 ISO), but still will give the best result when shot under the same circumstances as most exposures in the field.
This is why a camera histogram can tell you a lot, it prevents having banding because of underexposure or burn-out because of overexposure.
Terry is a master in experimenting and knows more about the technicalities of imaging than I do, as well as the use of Iris, which has an almost unlimited amount of processing features if you know where to find it.
Oh yes, nice shot, Narayan!
Cheers,
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