Adobe DNG Lossy vs. Lossless Comparison
The lossy DNG option
Adobe added a Lossy DNG option.
While not giving much information official, in some blog posts MadManChan from Adobe explains a bit further. All channels a seperately dithered to 8 Bit and then compressed like a JPG.
However the 8Bit reduction is done intelligently, so if an area need many levels near to each other (e.g. a blue sky), the 8Bit will be distributed to accomodate that.
Everything you love about raw is preserved though: white balance, DCP color profiles, your own noise reduction, highlight recovery all work.
Lightroom even handles those compressed files a bit different in e.g. the noise reduction (because of 8 bit dithering, the noise patters are different).
The compression is large and comes in handy archiving large amounts of image.
Possible losses in lossy DNG
What might get reduced in quality? Because of 8 bit and JPG like compression you would expect lossy DNG to have problems with higher noise ratios.
Then extreme edits might be a problem, e.g. pushing up brightness in shadows a lot. Detailed structures might show compression artifacts.
It should be better visible on high end sensors that deliver 14 bit quality.
At least all of this is typically a problem editing JPGs. But DNG is differently, as outlined above.
Test case
I used my Nikon D750 in 14 bit mode, and made two images that I pushed hard on edits and noise reductions.
I shot them both with low ISO and high ISO, and converted them one time lossy and one time lossless. Lightrooms was set to output them to lossless TIFFs.
These are the the image AFTER edited:
They are pushed hard in Lightroom 5: e.g. the first has Highlights -56 and Shadows +94. The second also adds +1.2 exposure, along with massive Shadows and Highlights edits.
Noise reduction on the high ISOs are in the 50s both color and normal sliders.
The compression gains in comparison to lossless DNG compressed versions differ a lot between high ISO and low ISO. High ISOs are (because of the noise) more difficult to compress. My two images were reduced by 39%.
Low ISOs are more uniform and easiert to compress, resulting in 71% reduction averaged for the images. So a really massive gain.
The first image has this nice detailed wall prone to how compression artifacts, the second has flat large surfaces that are prone to posterization.
Result
The image looked the same, no difference between lossy DNG and lossless ones on the first view. So I overlayed the images, extracted the difference and amplified it. This is the result, first the low ISO then the high ISO:
Interestingly, there is no difference on hard edges and in details, so the compression part seems to work very good. And generally low ISO shots are a bit more sensible than high ISO shots.
But looking maginfied at the error prone areas, so see no real visible difference. First is the blanked in the lower area, second is the area around the switch to the left. Both low ISO:
As you see, there is no visible difference. If you go 100% you see some very slight difference in gray steps, but not visible on a whole picture.
So I'd say Lossy DNG is absolutely safe to use even in extreme edit cases!
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