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Old 19-05-2013, 04:00 PM
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gregbradley
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Join Date: Feb 2006
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Hi Roger,

Interesting as I was just studying up on this myself recently.
There is a definite relationship between maximum dynamic range (showing shadows and highlights in an image) and ISO.

For most modern cameras I have seen graphs for maximum dynamic range is usually the lowest ISO it will do - typically ISO100.

Then it may stay stable for a little while, in the case of a Nikon D800E its much the same until ISO800 and then dynamic gain starts to fall.

You can see it in some images.

In nightscapes though you want the maximum ISO before ISO starts to deteriorate. That introduces a few measures. DXOMark uses an ISO rating where signal to noise ratio is at a certain level that is considered excellent image quality. Once ISO is raised above a certain point this signal to noise ratio falls and you now have less than excellent image quality. You can look up your camera and you will see an ISO rating.

See this graph. You can click on various camera models on the right to see the curve for your camera (I didn't see 60D though).

http://home.comcast.net/~NikonD70/Ch...adow.htm#D800e

It looks like ISO1600 is the optimum night sky ISO and higher than that you are simply losing dynamic range. Its much the same if you exposed at ISO1600 and then used exposure compensation in post processing to boost the image. Higher ISO simply multiplies the signal by a factor. Its not a totally simple relationship as manufacturers noise reduction enters into it as well.

So for example the Nikon D800E has an optimum ISO of 1600. This has been born out by my own testing. ISO1600 to 3200 seems good with this camera. ISO6400 is workable but you can see the data starting to stretch thin and dynamic range falling. By ISO12800 its becoming quite clear. DXOMark rates the D800E to ISO2910. So these graphs and DXO are not far apart in agreeing what is ideal.

That then means that the ideal exposure for your camera is what is required to get a bright exposure at that ISO and the lowest F ratio your lens can do without aberrations. So that means a tracked camera and a good lens is important. You can get away with ISO6400 panoramas but ideal is longer and stopped down and lower ISO for lowest noise, best signal and well exposed image that will take some processing without breaking down.

Greg.
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