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I have a Canon 550D with a 16GB SD card. I noticed that, when in P-mode, if I choose an ISO of 100 I can shoot 595 raw files on an empty card. If I choose an ISO of 200 this is 590. And if I choose 400 this number decreases to 583. Here below is the rest of the ISO vs. space

ISO __ | __ Amount of raw files that fit on 16GB
800 __ | __ 569
1600 _ | __ 556
3200 _ | __ 539
6400 _ | __ 510
12800_| __ 477

Does anybody know, why the filesize of one raw photo increases with the ISO?

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In general, photos at lower ISOs will have less noise. This means that they compress better (remember that RAW files have lossless compression) and so, on average, you'll be able to fit more images onto the card.

The other important thing to remember is that the number shown is only an estimate - how many images actually fit depends on what you take photos of and how much they compress.

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    The answer may have a degree of correctness to it but I feel there is enough more to it than that, that a more comprehensive answer is needed to fully lay things to rest. – Russell McMahon Oct 31 '12 at 8:41
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    @RussellMcMahon as far as I know that's pretty much it. Canon use lossless compression of RAW files. Images with more noise contain more entropy and thus more space to store. It's easy to verify this if you have enough RAW files on your computer. – Matt Grum Oct 31 '12 at 9:13
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    If you really need more details, I'd suggest the dcraw source as a good place to start; from my very brief look at the code, it looks like some sort of Huffman compression. – Philip Kendall Oct 31 '12 at 9:37
  • Would this answer mean that high ISO = more noise and hence would settle give a definitive answer on this question: photo.stackexchange.com/questions/6615/… – Saaru Lindestøkke Nov 1 '12 at 0:17
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    @BartArondson Yes, high ISO = more noise. If you're referring to Matt Grum's post on "lower ISO isn't always better", he's just saying that when there's too little light for proper exposure at base ISO, amplifying the signal in camera (through higher ISO) gives less noise than underexposing and recovering in post. So low ISO = low noise (assuming proper exposure, i.e. long shutter speed), high ISO = more noise, and base ISO + underexposing umpteen stops + recovering in software = most noise. – j-g-faustus Nov 1 '12 at 15:39
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This explanation over at PetaPixel goes into the "how and why" of ISO impacting file size.

Basically, more noise means more unique data and therefore a larger file.

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