RAW and noise reduction

[d3debian]

With my Canon EOS 400D i can switch on noise reduction with slow shutterspeeds.
Does it make sense to use it while shooting RAW?
Is the RAW-file unaffected by noise reduction at slow shutterspeeds?

[David]

Hi D3Debian - An interesting question for which other members may have a better answer than this. In short, the answer is yes, it does make sense to use this noise reduction method when shooting in RAW mode. I cannot strictly speak for the Canon 400, only for the 40D, but I think this feature is in all Canon DSLRs. As I understand the matter, long exposure noise comes from, mainly, thermal and source variations. After the exposure is complete, say 10 seconds, the firmware algorithm then takes another 10 seconds (or whatever) worth of exposure, but without opening the shutter. The firmware then combines this with the original data in such a manner to reduce the noise. I have no idea as to the nature of the algorithm that is used. Anyway, this processed data is then stored as the RAW (or whatever) file. I can't find any reference to this in my 40D manual, but then that's not unexpected for Canon.

I hope this helps.

Cheers

David

[d3debian]

I hope this helps.

Yes, it helps very much, thanks.
Now i know what happens in the time after exposure.
It creates a kind of repair-layer wich merges into the first one if i understand you correct.