Converting RAW to DNG! What are the advantages / disadvantages?

Question

I have Canon cameras and shoot RAW, but the question itself is surely also valid for Nikon, Sony or other brand-shooters.
When importing my pictures to Lightroom, I could convert them to DNG but didn't do this up to now, because I do not know what I loose. I suppose that I loose the ability to process my files with DPP, but I will not do that anyhow. I can appreciate the advantage of having the file in an open documented format, where even I could easily write a reader for it. So the answer should not be what I can easily find in the Adobe introduction into DNG. I have also the DNG specification in front of me, and I can read it, because I once wrote a program to read TIFF files. I understand that I could add the original RAW image stream, but I do not want using up the space wasted for this. I know, that all development parameters used for the specific RAW image can be written with the image file instead of having those stored in a database or a side-car-file.
But what I did not find until now a technical explanation about what happens during the convert. Who can enlighten me? Where do I find the missing technical explanations?
Thanks in advance!
Lucien.

Answer 1

Feierwoon wrote:
That wasn't the question... .
Based on the title of the thread and your initial post, it seems like your question had a lot of
"What are the advantages / disadvantages?"
in it, and only a little
"What happens at a technical level when converting"
But if you want more detailed info about conversion, and you have the aptitude to understand, I recommend doing what Lr5user-pt recommended:
Download the DNG SDK and dig in - all your questions will be answered (and if not, you can also ask DNG-specific technical questions in the DNG forum).
As far as my "sources" regarding the technical summary I presented, they are many and varied, none of which are the DNG SDK. In other words, my knowledge is based more on experience over the years - I don't know all the details under the hood..
Some experiments to consider doing:
* exiftool (-X) a proprietary raw file, then convert to DNG and repeat.
* open a proprietary raw in a DNG-supporting software of your choice and inspect metadata, then open the converted DNG in the same software and re-evaluate. Compare to proprietary raw opened in manufacturer's software, and what you see via exiftool.
Tom Hogarty (Lightroom project manager) and others who know have said: "DNG converter discards no metadata", and I believe them, but haven't verified for myself. Yet once proprietary metadata is in the DNG, most (all?) software will no longer decode it. A prime example is focus points - presumably they're there, but once converted, focus point feature is no longer supported. The amount of work required to support converted focus point data is unknown (to me), but as far as I know, no software has ventured to do it. If you want focus points, don't convert.
If you want the truth, I think for most people it's "not wise" to convert to DNG. Why? because unless you already know why you are converting (and need to convert to satisfy your goals), or you already know why you must NOT convert (e.g. so you can open the files in mfr.software, and see focus points..) the pros and cons are likely to be a wash, and now you have twice the number of raw files, unless you discard your originals, which would be REALLY "not wise", in my opinion. If you find the notion of "openly documented format" sufficiently compelling, then, ya know, more power to ya (I don't judge..), but there is no guarantee that DNG will outlast NEF (or..), and if it does, there will be ample opportunity to convert before you're left in the cold, unless you're stuck in a cave for a few decades, or are kryogenically frozen.. True, if all companies die, you may be able to write your own DNG converter (/reader), given the DNG documentation, but FWIW, NEF is also thoroughly documented (document is in freely downloadable SDK), and reverse engineerable (that's what Adobe and Phil Harvey does), so you could also write your own NEF converter, so not really a very compelling argument, not this year anyway - ask me again in a few decades..
If filesize is a main draw, then be sure NOT to save the hi-rez previews in your DNGs, otherwise you've just lost most or all of the filesize advantage. Also, you can strip previews from some proprietary raws (e.g. NEFs), to reduce filesize.
Personally, if DNG supported sidecars, I would be more likely to convert, because file-management/backup is only an issue if you "forget" about xmp sidecars (and I do not forget), and I'd rather have my xmp in separate ready-to-read xml text files, rather than embedded in binary files, but hey, that's me..
PS - I always use/encourage-others-to-use DNG when distributing raws to others for use in Adobe software, so xmp is not separate and can not get lost.
PPS - proprietary raws and dngs are both just glorified tiff files - raw data, previews, and metadata..
Don't get me wrong: I am NOT anti-DNG, and in fact - just the opposite: I am PRO dng, it's just that I wouldn't (don't) convert my own raws to DNG at this point. If you find the reasons to convert now are sufficiently compelling, you have my complete support and cooperation, fwiw..
Cheers,
Rob