Long exposure in camera processing.

tony64

tony64

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Tony
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I know this is going to sound like a daft question. But can anyone tell me why the camera takes as long to process the image after the shutter closes as it did to take the original shot? I am experimenting with some low light photography with an exposure of about 10 minutes, but this is taking 20 minutes between shots. (Using a canon 7d, bulb mode, wired remote timer). Is there any way to speed this up?
 
Ok, thank you for that. Do you know what the affect in IQ is when you turn this off?
 
tony64 said:
Ok, thank you for that. Do you know what the affect in IQ is when you turn this off?
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You may get hot pixels. It does not remove high ISO noise, of that's what you are thinking. It literally just maps and removed hot pixels (the occasional, random red, green or blue single pixels you sometimes get on very long exposures). They're a doddle to remove yourself though. General high ISO noise levels will be identical with or without long exposure NR.
 
Pookeyhead said:
You may get hot pixels. It does not remove high ISO noise, of that's what you are thinking. It literally just maps and removed hot pixels (the occasional, random red, green or blue single pixels you sometimes get on very long exposures). They're a doddle to remove yourself though. General high ISO noise levels will be identical with or without long exposure NR.
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Is that canon specific? Normally the dark frame subtraction will also remove all dark counts, leaving only the shot noise from the dark frame.
 
I am trying to keep my ISO as low as possible (low 100's) by extending the exposure time. I'm guessing the best way to see the difference is to take two shots and compare them afterwards.

Thanks again to everyone for you advice.
 
You can also take a matching dark exposure yourself later on. This will show the consistently hot pixels (take a bunch in case you want to see the always flaky ones as they tend to stay hot). Just make sure you've got no light leaks into the camera, and that the temperature is roughly the same (thermal currents are what gives rise to the general rise in noise as opposed to the hot pixels). And also be aware that Canon appear to do some in camera processing even on their raw frames which can make direct comparisons of dark and normal frames less trivial than it should be (if you want to see an article on the whole gruesome business try http://www.stark-labs.com/craig/resources/Articles-&-Reviews/CanonLinearity.pdf).
 
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