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What iso setting to use?

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Pondering this problem, it occurred to me that on a DSLR what probably happens is that all the pixel values are bit shifted for each full stop increase in iso. This would make a value of 1 at iso 200 2 at iso 400, 4 at iso 800 and 8 at iso 1600 (available valuse on my Nikon are only 200 to 1600 iso).

The binary equivalents are 0001, 0010, 0100 and 1000.

I wondered if anybody knew for sure if this is the case and also wondered if this is the reason why a lot of my pictures appear to be posterised after processing.

If this is the reason for the posterisation I assume that I need to go lower on iso and longer on exposure times?

Captain Chaos

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As the other guys CC. Usually go with ISO1600 to get the detail and ISO 800 to clean things up a little. All last nights efforts were at ISO800 because they were relatively bright objects.

I think once i can autoguide and achieve some proper long exposures I may try dropping to ISO400. Thats a way off though.


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Each pixel produces a signal together with some underlying background noise (from the imaging devices electronics). The noise is the imaging equivalent of the audio hiss heard when a hifi amplifiers volume control is turned up.

In low light, the weak signal is often only slightly greater than the background noise (a low signal-noise ratio). Raising the ISO amplifies both signal and noise which is why you see mottled grain/colour in the final image.

Hope that helps :lol:

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It has got to be a compromise between ISO and exposure. Short exposure, less noise, less detail. Long exposure, more noise, regardless of ISO. Although they are not as simple as that. Also ambient temperature can have an effect, especially on long exposures.

Only by experiment will you find the best balance for your setup. it would be worth while trying different combinations, particularly if doing DSO's

I know that most use the 1600 ISO to great effect, and this allows some great captures. if you have a fast scope, i.e. f5 or f4, this would be OK for shorter exposures on DSO's, if they are large enough to see, but pretty useless on planetary. Although stacking would have to be used even for this arrangement.

Similar problems arise with film, the higher the ISO, the larger the grain, so you have the analogue equivalent of noise.

I think it is something that cause great debate for ever more, until the noise side can be controlled at a reasonable cost.

thats my twopence worth.


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