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PostPosted: Sat Dec 31, 2011 4:37 am 
I've noticed on my Canon 550D that if I'm doing long exposures, say 30 seconds, that they take AGES to save and was wondering why.

Just done a quick test and its taking about 30 seconds to save a 30 second exposure.

I have a fast-ish card in there (Scandisk Extreme 16Gb 30MB/s x10)

Anyone know whats going on?


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PostPosted: Sat Dec 31, 2011 9:03 am 
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Joined: Mon Jan 18, 2010 3:52 pm
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Location: The Netherlands
The camera substracts a Dark frame to reduce noise. Turn Long exp. noise red. off in the custom menu to disable it.

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PostPosted: Sat Dec 31, 2011 9:37 am 
Ahhhhh, cool, thanks Ruben, that info is much appreciated. I'll do a couple of trials to check the difference to see if the wait is worth it


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PostPosted: Sat Dec 31, 2011 1:09 pm 
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Longer exposures with higher temperatures and higher ISOs, yes, there will surely be diffrences. But, 10 degrees/15 degrees, 100-200 ISO, no, it wont be that impressive the Dark frame substraction gives you.

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PostPosted: Sat Dec 31, 2011 1:24 pm 
Actually I read somewhere after googling 550d Long Exposure Noise Reduction following your answer that the noise reduction works best on lower ISO's and that with higher ISO's it can actually make it worse.

Here in the middle east if it gets below 20 deg its considered a bit chilly, its been down as low as 15 recently and everyone has been complaining bitterly!! Temperature induced noise and whether there is anything I can do about it is something I'd like to work out.

Here's the link re higher ISO's and increased noise with LENR http://www.imaging-resource.com/PRODS/T2I/T2IA5.HTM

Quote:
Long Exposure noise reduction can be performed for exposures one second or longer, and works by taking a second "dark frame" of equal duration with the shutter closed, and then subtracting it from the first frame. This reduces or eliminates most noise generated by the sensor during long exposures at low ISOs, but can make noise worse at higher ISOs (at ISO 1,600 and above)


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PostPosted: Sat Dec 31, 2011 2:02 pm 
Maybe because its taking a long exposure. You can't wait a minute for a photo??


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PostPosted: Sat Dec 31, 2011 2:07 pm 
Palkaboy- ha ha, of course I can wait a mo. The query was because I was wondering why and it turns out there was a reason (on exposures longer than 1 second its actually taking 2 exposures!!) so the question was worth asking!!


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PostPosted: Sat Dec 31, 2011 3:29 pm 
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one thing to remember would be to disable it if your doing extremely long exposures :P
i did a 17minute exposure with the 500D i had earlier, and it ended up taking another 17minute exposure for the noise reduction.
it ended up using probably 75% of the battery.

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PostPosted: Sat Dec 31, 2011 3:52 pm 
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Generally I would turn it off, unless you're only doing "shorter" long exposures where it doesn't have much impact on shooting time. Hot pixels are usually handled quite well with modern raw converters anyway, and if needed for special applications (astrophotography) you could separately do a dark frame subtraction instead.

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