Why convert RAW files to DNG?

A 2007 survey of photographers using Photohop and Lightroom found that something like 40% of those photographers are now converting their RAW images to Adobe's DNG format. I do this too, and I thought it might be helpful to tell some about what I know about the format and also describe my workflow.

When I began shooting digitally I didn't work with RAW files at all. Mainly that choice was driven by the equipment I was using. Even though my Fuji S3 could capture images in their RAW format (.RAF), the camera took almost 12 seconds for the data to clear from the buffer as it was being written to my CF card. During that time, you could not make another image. Perhaps if I was shooting a landscape or was in a situation where I didn't need to worry about shooting more rapidly it would have been okay to shoot in RAW. But shooting news assignments and events and weddings that just wasn't an option. So for a long time I set the camera to make JPEGs of all my images, and I was fortunate that the camera produced some beautiful images in that format. I still use the S3, even though I have added several newer bodies to my arsenal.

After I got my Nikon D200 I started shooting RAW files (Nikon calls them .NEF files) and I quickly found that it opened up some interesting possibilities in my workflow, and that it also created some new headaches for me. Programs like Adobe Lightroom seemed to work a little more smoothly and quickly with RAW files, in part because it was designed in large part to do just that. And of course it integrates with the Camera Raw processor for Bridge and Photoshop, letting me apply or undo settings across the various programs. But my JPEG files had only been something like 4 or 5 megabytes in size (when unopened), so I could shoot a whole wedding with just a couple of 1-gig flash cards. The .RAF files from my Fuji S5 clock in at a whopping 25 megabytes per image, and I quickly found myself having to buy 4- and 8-gig CF cards as well as budgeting for a number additional hard drives for my computer each year. Suddenly, having a few terabytes didn't seem like such an extravagance anymore.

My collection of CompactFlash cards grew once I started shooting RAW instead of JPEG.

Fortunately, you can go a long way toward solving this storage problem by converting your files to DNGs, and you can pick up some other benefits along the way.

If I could cite no other benefit to using the conversion, I would still be pleased with this part of it. You can see this illustrated in the screenshots below, showing files that I shot at a wedding late last year. I've renamed the files, but I am being honest about what the information reveals.. the DNG files created from the Fuji S5 are nearly 40% larger than the original .RAF files. DNG's made from the Nikon D200 show a similar space-saving effect.

The .raf files from my Fuji S5 are 24.5 MB each,
but converting them to .dng files means they take up much less space.
Even though I renamed these files, they are each from that camera.

What accounts for the difference in size? Well, that's not entirely easy to explain. NEF and RAF files, as well as Canon's CR2 and many other RAW formats, are called Proprietary Files, meaning they belong to those manufacturers. They create these formats to work with their specific camera models, and they want you to use their specific software to work with those files once you get them in to your computer. That way, your Canon images that you made on your Canon camera all stay in the Canon family, and you end up buying more stuff from Canon. (Substitute your own brand anywhere in that last sentence.) In order to keep them "proprietary," they don't tell other companies (like, say, Adobe) all the stuff that's in them.

There is some good speculation for some elements to account for the size change. For one, it seems common that there are several levels of preview files (generally JPEGs) of the main image that are included along with the data. That's how you can see the photo right away on the back of your camera and once you start loading them in to various other programs on your computer. Because software like Lightroom builds new previews from the image data and stores it in a separate library file, you can get away without having that stuff embedded in the original.

Another possibility (and again, all we can do is guess, 'cuz the companies ain't sayin' nothin') is that there is extra data in the file that allows specific functionality with the maker's own software. For example, Nikon Capture lets you work with specific areas of an image to lighten a subjects' face without having to adjust the entire image. At present that functionality doesn't exist in Adobe Camera Raw or Lightroom. It's a cool feature, frankly, and I wish that Adobe would figure out a way to add this to the already impressive lineup of features that they afford.

So that could be an argument for not converting your RAW files to DNG and just using, say, Nikon Capture or Fuji's HyperUtility for their files. But to my mind there is a slight flaw in that logic, and it's based on my experience with software. Even if they say they will support software and cameras down the road, it's not always the case that companies actually do that. To cite extreme cases, I sometimes like to say that no one is listening to their eight-track tapes anymore, except for the creepy guy you stand next to in line at the Taco Bell late at night.

A few years down the road, that proprietary file you have sitting on your hard drive could quite possibly be just a useless mass of 1's and 0's that you can't access. This is where conversion to DNG once again saves the day.

Adobe developed the DNG format to convert RAW files and has made the coding openly available to anyone that would like to take advantage of it. Even though they thought it up, they aren't trying to "own" it. And the recent history of open source software has shown that this is a pretty good model for sustainability, because as new situations arise there is a large field of determined geeks willing to write new code to deal with them. It will not surprise me even if some of the secret proprietary information is eventually decoded by some of these same geeks (don't get me wrong; I love geeks, and depend on them, and secretly long to be accepted by them) so that you might someday be able to take advantage of those specific features. The chance of that (decoding the secret stuff) happening is probably small, whereas the chance of the first part (geeks constantly enhancing and supporting open-source software) is pretty good. So down the road I'm confident that I will be able to open, view and work with my DNG files, and enjoy making adjustments to them in future programs while I listen to my 8-Tracks and eat Gorditas.

Next up: some workflow thoughts on handling and storing DNG's and RAW files.

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