RAW vs DNG

Among the things I often find myself writing about in online photo fora is the question of whether or not you should be leaving your photos in a native RAW format or converting to DNG.

Some background:

  • RAW formats are (generally proprietary) file formats for digital cameras which essentially record the camera settings (ISO, shutter speed, focus mode, etc.), current metadata (time, location if available, etc.), and sensor data, all captured at or near the moment the shutter is pressed.  RAW formats are often considered to be superior to "developed" formats like JPG or PNG because they have not been processed by the camera in irreversible ways.  However, RAW formats are not intended for viewing, just as film negatives are not -- they must be processed in order to create viewable images.  Common raw file extensions include .NEF (Nikon), .CR2 (Canon), .ARW (Sony), .RAF (Fujifilm), and .ORF (Olympus).
  • DNG (Digital Negative) is a patented, open, royalty-free file format developed by Adobe, extended from TIFF, intended for the storage, manipulation, and archiving of photographs.  It is intended for use as an in-camera RAW format like any proprietary RAW format as well as an archival (long-term storage) format and for use by imaging software like Photoshop and Lightroom (both produced by Adobe) and software from other vendors.

Pros

Reasons I often see for converting to DNG include (but are not limited to):

  1. Long-term viability of the format (the concern that native RAW formats will become unusable in the future)
  2. Internal validation of DNG file data
  3. Space management (DNG files allegedly being smaller than native RAW formats)

Let's take them one at a time:

1. Is DNG a better bet, long-term, than RAW?  Maybe.  Depends upon your RAW.  Ultimately this question is unanswerable, but if you are using a popular camera brand (Canon, Nikon, Sony, Fuji, etc.) that is already supported by Adobe, odds are you don't have a lot to worry about.  I can't find a single article claiming that Adobe has removed existing RAW support from ACR or Lightroom, ever.  Furthermore, there's no real incentive for them to do so, assuming the RAW file is already in existence, supported, and not changing.

And if Adobe were to eliminate RAW support for some files, there would be warning (unless Adobe just vanished from the world somehow) and you'd have plenty of time to convert from the soon-to-be-eliminated RAW format to DNG when that warning came.  And you'd probably have a faster computer then than you do know.

There's an alternative view of this, however, which has to do with archiving images instead of you working on them.  Suppose you expect that, 100 years from now, people will think of your images the way we now do about early titans of photography like Ansel Adams and Dorothea Lange?  That people might want to comb your catalog for images you never processed or re-process images you did?

In that case, I admit that DNG might have some advantages.  But it's far from certain that DNG will still be extant and viable in 100 years.  And without trying to put anybody down, given the number of images being created annually these days, I seriously doubt that future photographers are going to be desperately perusing 100 year old Lightroom catalogs for source material.

2. Is DNG a better, more robust, file format than RAW?  Again, maybe.  We know that DNG includes some internal checksumming.  We don't know whether or not RAW files do, because the nature of propriety files is that we don't know exactly what's in them.

This question comes down to: how much you trust your computer hardware and software to properly copy bits from point A to point B; how much you trust your storage hardware to maintain bits in their original state; whether or not you maintain multiple backups; and what you consider the necessary lifespan of your storage systems to be.

Modern computers are far more reliable than early ones.  Modern hard drives and SSDs are far more reliable than early ones.  And we have more options for backup systems now than we ever did before.  None of that is a guarantee that we're not going to lose data.  But honestly, if you are worried about your data, make sure you have good multiple backups (and test your restore pipeline), don't worry about whether or not there's a checksum in your RAW file.

3. Are DNG files reliably smaller than RAW files?  Who cares?  Really.  Most of the arguments I've seen about DNG files being smaller than RAW results from a comparison of DNG files with uncompressed RAW files, which is ridiculous.  Most cameras support lossless compression in their RAW files, and the comparisons I've found of compressed RAW files with DNG indicates no significant difference in size.

But suppose there is a significant difference in size?  Again, who cares?  Managing terabytes of information already comes with the job of digital photographer.  If DNGs are 20% larger or smaller, is that really a deal-breaker either way?  I don't think so.

Overall, I do not see the usual arguments for DNG to be compelling.  The most compelling of them is the idea that your RAW file may someday be deprecated, and IMO the solution to that problem is to convert images then, not now.

Cons:

Typically this sort of article would now list a bunch of reasons why conversion to DNG is a bad idea.  I will do so but only briefly, without the length of explanation provided above, because without a compelling reason to switch to DNG, why worry about reasons not to?

  • Converting to DNG requires resources, most notably time.
  • If you chose (a relatively rare circumstance, I imagine) to preserve a copy of the original RAW data within the DNG, you are roughly doubling the size of your files, and while I argue above that 20% one way or another doesn't make a big difference, I'm thinking 100% bigger probably does.
  • DNG does not preserve all camera specific information, and once you lose that info, it's gone forever, unless you also preserve the original RAW data, either as a separate file or as part of a DNG.
  • Not all programs fully support DNG, it's been far less widely adopted than Adobe had hoped.  Does that mean it won't ever be widespread or viable?  Not necessarily, but we're not really there yet.  In particular, Phase One Capture One Pro seems to be a very popular alternative to Lightroom which has a significant feature (better camera profiles for importing and converting RAW files) which is essentially negated by pre-conversion to DNG (because DNG files imported by Capture One have their camera profile changed from the original to "Adobe Standard" which is apparently difficult to walk back, although I suppose that might change in future versions).

Finally:

I don't see compelling reasons to convert all my photos to DNG upon import.  That doesn't mean I'm unhappy when I use Lightroom to create a panorama or HDR image in DNG.  I have nothing against the format, I just don't see it as a solution to any significant problem I have.  And if I had a camera that used DNG as its native RAW format I'd be fine with it.