Due to the fact that lossy compression (which basically means that data is thrown away from the RAW file in order to keep the file size down) is applied to the RAW files in all of the A7 cameras (and possibly other Sony cameras too), artifacts start to show up. These artifacts are visible in certain parts of the image when lifting the exposure/shadows by a couple of stops, but sometimes even show up when hardly any image processing was done.
1) A quote from an article on the RawDigger website:
"All that is necessary for the artifacts to threaten the quality of the final image is a combination of high local contrast and a flat featureless background."
2) A quote from a forum post by Fred Miranda on the Fred Miranda forums:
"I was able to replicate a situation where the compression is visible. When shooting a high contrast subject where the shadows are very close to pure black. When trying to recover the shadows, artifacts become very apparent."
One might ask if this issue is caused by the lossy compression that is applied to the Sony RAW files. Let's compare an ISO100 Sony A7 II RAW file to an ISO100 Nikon D750 RAW file (which both use a very similar Sony EXMOR 24.3MP fullframe sensor). This can easily be done by downloading the RAW files from the "New studio scene comparison tool" on the DPReview website.
You can find a quick comparison I've made, right here:
Sony A7 II RAW vs Nikon D750 RAW
As you can see, the artifacts are not there in the Nikon D750 RAW file which is merely 6,5 MB bigger in file size (24 MB for the Sony A7 II RAW file vs 30,5 MB for the Nikon D750 RAW file). Because of this, it's (very) sad to see that Sony is crippling the performance of their sensors in the A7 cameras by applying lossy compression to the RAW files.