DNG Conversion Performance

I had, for a number of years, been converting my raw images to dng format. Two years or so ago I stopped doing that. Why? Time. I was taking a workshop and using my underpowered MacBook Air to review images at the end of the day. It took a very long time. Since I wanted to get some sleep, I stopped converted images. The Lightroom import process took less time. This doesn’t save any time for the preview build but it did reduce the time to import. In addition to converting to DNGs, I also always have used Lightroom to rename and move the images from the SD card to my disk.

Recently I’ve been reexamining this idea. Part of my motivation was that I’m not anxious to subscribe to Lightroom and Photoshop. One advantage of using DNGs is that my older version of Photoshop doesn’t complain about reading the raw files. Another possible advantage mentioned for DNGs is the XMP files are not required. That’s also mentioned as a disadvantage because the XMP files are small and could take less time to back up.  I’m not sure about that.  A short investigation seems to say that the DNG file is not changed until it’s exported.  For example, the GPS data is in Lightroom but not in the DNG file on disk. So it would depend on what your workflow is. Finally,  DNGs are smaller than the raw files for most cameras. In my case, the Fuji raf files take 33MB while the DNGs are around 23MB. Given that storage is cheap, I don’t see that as a winning reason to change.

Unfortunately, the time to for Lightroom to convert into DNG format is prohibitive. Here’s one example:
I used Lightroom to import and convert 59 raf images to DNGs. The source was my SD card reader while the destination was a NAS. ( Using a NAS probably results in slower times than using an internal drive. ) It took 1:54 before the import was done – that is, I could see all the thumbnails. It then took 5:57 to build previews and convert to DNG. It appears the DNG conversion is interleaved with the preview building. The total time was around 8:00.

For comparison, I used the Adobe DNG Converter (10.2) to convert the same images. Again, the source was the SD card and the destination the NAS. I renamed the images at the same time. It took 45 seconds to move and convert the 59 images. (I didn’t really believe this so I repeated the test. Same result. Later I did the same process with more images. It is consistently around 750 milliseconds per image to convert and move.) I then added those images to Lightroom. I could see all the thumbnails in around 10 seconds. Then the preview building (Std) took another 3:40. One ‘feature’ of this is that the DNG converter doesn’t make a second copy of the raws; you can do that in Lightroom. My solution was a simple command line copy to make a back up copy of the SD card. That can be done after the import or any other time that’s convenient.

The total time was significantly faster than using Lightroom. So, at least for now, I’m back to using DNGs but not using Lightroom to do the conversion. Update 3/27/18 I realized I had not tested the case of just importing raw files without converting to DNG. Turns out, that’s much faster also so if you really don’t want to use DNGs, you can still save time by copying the files, renaming and then add to Lightroom.

Want to read more? Here’s a couple of places to start,

DNG: The Pros, Cons, and Myths of the Adobe Raw File Format

Why I Don’t Convert To DNG