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Tuesday, February 26, 2013

DNG File Format & DNG Converter

In this excerpt from his book, digital master photographer and Photoshop Alpha-tester Jeff Schewe demystifies the DNG format for today and for the future

This Article Features Photo Zoom
Another new function of DNG is the ability to apply lossy compression to make DNG file sizes much smaller and even downsample the actual DNG image file. Previous versions of DNG Converter allowed adding lossless compression for some size savings, but this new feature can make really small DNG files. Figure 4 shows the various options available in the DNG Converter Preferences dialog box.

Figure 5: Lossy DNG File Sizes
Original DNG with Fast-Load Data 105.7 MB
DNG with lossy compression 29.1 MB
DNG with lossy compression sized to 20 MP 29.1 MB
DNG with lossy compression sized to 3 MP 1.8 MB
DNG with lossy compression sized to 1 MP 541 KB

The ability to add lossy compression and downsampling to a DNG file is intriguing, but a bit scary. Here's why it may be scary:

 Lossy compression loses something from the original. The lossy compression it uses is JPEG (about level 10, but that could change). In order to process the image, the image must be demosaiced and saved as a linear DNG file. So, it's no longer a true raw file in the classic sense, but it's still in a linear gamma, which means it can be processed in Camera Raw and Lightroom as a raw file with excellent results.

 Downsampling reduces the original resolution. Although the lossy compressed DNG is editable, it will no longer have the full resolution of your original file. That's not necessarily a problem as long as you take great pains to make sure you have a filenaming convention that will never save a downsampled DNG over your original DNG.

But the lossy compression and downsampling can be very interesting when it comes to the ability to transmit or exchange DNG files that can still be edited in Camera Raw and Lightroom. To get an idea, check the table (Fig. 5). The image I converted to DNG is the same rusty, old truck (Fig. 1).

As you can see, there's great potential. Unfortunately, we're really just at the beginning of a lot of the potential for DNG with lossy compression. While the compressed DNGs retain the capability for editing, moving the XMP settings data back and forth between the original DNG and the lossy DNG hasn't really been worked out yet. The concept of some sort of proxy DNG would make it possible to store your originals while transporting your downsized compressed DNGs and retaining the ability to update the new editing back to your originals. I have no doubt that's what's in the minds of Thomas Knoll and the Camera Raw engineers, but at this point I don't know what's going to happen.

You can order Jeff Schewe's book
The Digital Negative: Raw Image Processing in Lightroom, Camera Raw, and Photoshop at Amazon.com You can see more of his work on his website at schewephoto.com.


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