I'm evaluating the DNG lossy compression and discovered something strange. Would appreciate it if someone would duplicate my test and tell me if they get the same result, or if there is something flawed in my test procedure. It appears to me that "Lens Profile Corrections" are applied differently between lossy and lossless DNG files.
I take a normal, uncompressed DNG file, load it in ACR (7.1) and save a copy with "Use Lossy Compression/Preserve Pixel Count." I then load both of these in Photoshop, and layer one on top the other. Usually there is no visible difference, so I put the top layer in Difference blend mode, display the histogram, and read the StdDev value. I get StdDev values between 0.6 and 2.5.
That's quite a range, but I assumed variables like noise would account for some difference. A high ISO noisy shot would likely compress differently that a clean, low ISO shot. But with more tests and closer inspection I noticed that my wide angle shots were showing the larger differences (StdDev > 2.0). So on some wide angle shots I put the layers back in normal blend mode, set the display to 500% or greater and toggled the layers to look for visible differences.
To my surprise, it looked like the images had different registration. Portions of the image appeared to be shifted 1 to 3 pixels. And the shift appeared to be greater near the edges than in the center. So I converted some wide angle images with the Lens Profile Correction off. Without lens profile correction the visible shift in registration disappeared and the StdDev value dropped significantly. With lens profile correction on, I get StdDev values around 2.1. When it's off I get StdDev values around 1.2 on the same images.
I have not tested other variables yet, but I'm confused why lens profile correction would be different on a lossy vs. lossless DNG file.
My tests were on Canon 5D3 images with the 24-105 lens, using ACR 7.1 in Photoshop CS6 on Win7.