I am looking for a compression software to work on folders full of RAW (CR2) files with the following characteristics:

  • Lossless
  • Excellent compression ratio (>70% would be nice)
  • Reasonable compression time (lets say ~10min for 1GB)
  • Works on Mac OS X

I primarily need it to be able to send my photos to a remote backup when on the road.

I was looking into FreeARC (no OS X client), Rawzor (not great compression), ZPAQ (very slow). I was hoping that maybe someone knows of anything better?

Thanks a lot!

  • 1
    \$\begingroup\$ A CR2 file is already compressed. At the full pixel count of ~23.4mp megapixels, and 14-bit precision, an uncompressed 5D III RAW image from a Canon sensor would be 41mb, while the average CR2 file from a 5D III is about 26mb, meaning a compression ratio of ~36%. You aren't going to get much more compression out of a CR2...not losslessly. \$\endgroup\$
    – jrista
    Commented Apr 12, 2013 at 15:11
  • \$\begingroup\$ ...and that's just the sensor data. There's also the as-shot settings and EXIF data (which are trivially compressable) and three lossy JPEGs living in TIFF containers (which are not). \$\endgroup\$
    – user2719
    Commented Apr 12, 2013 at 19:42

1 Answer 1


I wouldn't hope for much; CR2 files are already compressed, so losslessy compressing the file further would entail decompressing the existing data then using a marginally more efficient algorithm. The corresponding expander would than have to decompress your version of the file and recreate the CR2 using the original compression (and byte alignment, etc.). You might get a few percent difference for a lot of processing at both ends, but 70% isn't going to happen. And because the files are already compressed, trying to compress them again as-is will likely result in larger files on average.

  • 2
    \$\begingroup\$ I've tried several compression utilities in the past, even the really slow ones that are supposed to give the smallest file sizes. I've yet to find anything that reduces the size of the file by even 10%. \$\endgroup\$ Commented Apr 12, 2013 at 12:54
  • 1
    \$\begingroup\$ Indeed, I have tested FreeARC on Windows on my CR2 files and got only 5% smaller size... \$\endgroup\$ Commented Apr 15, 2013 at 16:13

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