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I do benefit from higher ISO's but that's because I have really dark skies.
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You are right. Some times we need high ISO because the target is very faint. But it is not a LAW.
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you have increased signal but noise has also increased
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Maybe the most important is that some areas or details of target would be missing inside the background glow.
M42 isn't a good example, I know. But you can see that you loose all information and details of Trapezium.
Aniltak has nebulosity around. That nebulosity will be lost too.
Many people ask me why I use low ISO, sub with no more than 4 minutes. Well those photos show that: in
my special case the ISO 400 and 800 are the best for most targets. Not all targets.
More than 4 minutes... and I will capture only the sky glow !
The camera used, 350D, is more sensible, because it hasn't the loss in IR filter. But it would be very similar to camera 1100D.
Did you see my photo published here from M 42 in another message ? With my Canon 350D I don't believe I can do more than 2 minutes for subs. It will explode the target !