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Old 08-04-2012, 03:10 AM
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luigi is offline
Join Date: Jun 2010
Posts: 438
Originally Posted by Terry B View Post
For astro imaging, why does a high ISO make any difference?
All it does is change the in camera amplification ie the gain of the CCD. This potentially reduces the dynamic range of your image. All astro photos should really be taken as RAW and amplified if needed afterwards anyway.
The Signal to Noise ratio improves as you increase the ISO. This is because the sensor read noise is not amplified as the signal is.

So as long as the dynamic range is enough you should shoot at the highest possible ISO for a given exposure time.

For most DSLRs this stops making sense beyond ISO1600 but up to 1600 the higher the better.
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