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Old 16-07-2014, 08:57 PM
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Amaranthus (Barry)
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Join Date: Jan 2014
Location: Judbury, Tasmania
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In DSLRs, the ISO is changing the gain, so as to get the maximum range out of your pixels. If you are taking short subs, you'll need a relatively high ISO to distinguish different levels of pixel signal, since the ungained signal differences will be small. With long exposures, you've got more signal and are using more of the full range of your pixels. So you don't need such a high ISO.

Basically, the longer the exposure you take, the lower the ISO you should be setting. But remember that ISO cannot give you more signal, it can just stretch what exists over your dynamic range.

You should aim for the longest exposure you can get without bloated or trailed stars (without guiding) or otherwise (with guiding), you need to think about background and saturation.
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