Quote:
Originally Posted by bloodhound31
If I take a photo of the sky for 5 minutes at ISO 800, then another for 10 minutes at ISO 400, would the result be the same? Would the image be smoother/less noisy at the lower ISO with the longer shutter speed?
Baz.
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The ten minute exposure has twice as much information, so will be better! Of course, if it's overexposed then you shorten the sub-exposure time.
The more objective question is whether a five minute exposure is better at ISO400 or 800 and the answer is it makes bugger all difference. All you are really doing is changing the 'gain' setting on the camera.. the photons/electrons being counted at the sensor are the same.
Somewhere around ISO400/800 is about unity gain for most DSLRs (in 14 bit RAW), ie. at that point every electron is being converted to a data number in the file so increasing ISO above that does not make much difference, although you do start to sacrifice well depth/dynamic range.
http://philhart.com/content/astropho...xposure-length
Check out ClarkVision site for really in-depth info:
http://www.clarkvision.com/articles/...mance.summary/
cheers
Phil