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01-10-2013, 07:34 PM   #16
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Thanks guys

01-10-2013, 08:01 PM   #17
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Long exposures with image sensors are prone to dark leakage current or thermal noise. Typically, for exposures of 1s+ (0.3s+ K20D), Pentax cameras take a 2nd dark frame (without the shutter open) and then subtract the common elements (i.e. the hot pixels). This can be a pain when you want to take a lot of long exposure in a session, e.g. for fireworks, as the effective wait between shots is doubled. A work around is to turn off DFS (Dark Frame Subtraction), called "Slow Shutter Speed NR" in Pentax cameras, take 1 DF with the same shutter speed manually during the session, and then do your own DFS afterwards in your raw converter e.g. RawTherapee.

http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.html#thermalnoise

Image noise - Wikipedia, the free encyclopedia

Here is an interactive Java tutorial on image sensor noise. If you set Dark Current to the 10-100 electrons per sec range, and then increase Exposure Time above 1s, you will see how the thermal noise component, which is insignificant at fast shutter speeds, starts to become significant within the total noise.

http://learn.hamamatsu.com/tutorials/java/noisegraph/


Dan.

Last edited by dosdan; 01-10-2013 at 08:41 PM.
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