Canon’s mRAW, sRAW formats and DNG

Do you use the mRAW or sRAW capture options on your Canon dSLR to save space?  If you do, there’s a chance your good intentions are backfiring.

If you leave your files in the native Canon .cr2 format, there’s nothing to worry about.  But if you’re like me and always convert everything to Adobe’s DNG format, you’re actually increasing the file size.

The reason has to do with the way the data is stored.  Unlike a full RAW file, the sRAW and mRAW files are not true RAW files.  That is, they’re more like a super-powered .tiff file in that the pixels are a full mix of all three color channels.  (What techies call “mosaiced data” versus the “demosaiced data” that comes from a true RAW.  For a deeper technical understanding of this concept, check out Wikipedia’s article on demosaicing image data.)  When you convert this data to DNG, it cannot store it as efficiently as it can store a true RAW, and the file size increases significantly.

Consider this example.  I took one photograph with my Canon 5d mark III set to mRAW.  I then made two copies of that file for three total.  One I left in its native .cr2 format, one I converted to DNG, and the third I converted to lossy DNG.  Here are the resulting file sizes:

CR2 (original file):  21.3MB
Lossless DNG:  31.2MB (46% increase)
Lossy DNG:  3.8MB

As you can see, the increase in file size is significant.  But there’s hope!  Adobe’s Lossy DNG format sees huge gains from the lower resolution file.  I know “lossy” sounds scary, but in my experience the files are robust and retain a very high degree of quality.  Next time you’re out shooting for fun, try it out and see if you can detect the loss of quality in the Lossy DNG files.  Perhaps it’s a good solution for you, and it would mean even more storage gain.

[Notes – DNG Conversion Settings]
All of the above files were created with the DNG converter set to embed a full 1:1 preview.  This was done to create the most fair file size comparison, since the .CR2 file contains a 1:1 preview created by the camera.  If you choose to completely disable the preview, you’ll see even smaller file sizes.

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