I'm not convinced it's necessarily down to lossy raw. I suspect its more down to how Sony sensors work, in conjunction with the processors in the camera manufacturers. My A7RII is my back up camera. My 'main' camera is a Pentax 645Z which has a Sony sensor. Before that I had both the D800 and the D800E alongside the Kodak CCD sensor 645D. I always shoot RAW and with the Nikons chose lossless compressed.
Banding in the highlights, particularly in a twilight or dawn sky has been a common theme with all the Sony sensored cameras, though to a lesser extent in my experience with the Nikon files - though I have changed the way I process images since I had the two Nikon bodies. Some sensors are able to deal with shadows or highlights in a different way. My experience with Canon 5D series (I, II and III) is that recovering detail from highlights is generally easier and a more painless with the Canon sensors, but the pain is felt in the shadows, which prompted my move to Nikon.
Even with the 645Z I get artefacts in highlights where blue twilight/dawn sky meets the emerging or disappearing sun. The cyan and blue errors have a tendency to posterise if pushed far or even at all in some instances. I have found this with all the Sony sensor cameras - so D800/E, Pentax 645Z, A7R. The conditions have been not conducive to these conditions since the UK summer this year is most like Autumn, so I don't know how it will respond, but I'd put my money on needing to shoot to protect highlights to keep this element at bay. I can get more than 4 stops out of the shadows with the 645Z with little or no IQ penalty, and my experience with the A7R was very similar.
I know that many pro photographers expose to the right, and advice people to do this. With Sony sensors, I do the opposite.
My advice? Keep your box and accessories, don't trade in before purchase if you are particularly concerned, and get into the camera and try to learn how to use it best. Remember that processing is only part of the job, and it's also down to how the images are shot. Mine will look dark on the VF and EVF, but I know from experience that I will be able to eek out detail from there, but that there is much less wriggle room in the highlights on any camera that I've had with a Sony sensor. Check what your return rights are legally and by policy with your retailer. (In UK you can return something unopened within 7 days if it has been ordered via distance selling. There is no right in the UK when buying on the high street to return something unless there is a manufacturing fault, though some retailers do allow it.) You might have problems convincing a sceptical UK retailer that this is a fault rather than a 'feature'. The sales of goods act in the UK is not necessarily the most consumer friendly.
YMMV, but that's my experience...
I'd like the team to get proper 14bit lossless ASAP. But I will see how it behaves - hopefully the promised sun at the weekend will materialise and I can get some twilight shoots in...
Some from Milford Sound:
Sony A7R variety of 16 - 35 f/4, 24 - 70 f/4 and 70 - 200 f/4
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