Aye, the 7D's read noise is quite a bit lower (although not nearly as low as an Exmor sensor, which ranks in at 2-4e-). I think the 7D has around 17-18e- read noise at ISO 100, which is roughly HALF that of the 5D III. I am honestly rather baffled by the read noise of the 5D III sensor. For all the progress that is apparently possible in the area of read noise, and how important it is to maximum DR, it really surprised me to see it JUMP from the 5D II's 27-28e- to over 33e-.
Another elementary question: Can there possibly be any benefit of increased read noise? I assume not, but it is very odd that it would increase on the 5DIII over the 5DII. Also, is read noise the same as visible noise in an image? If so, how does the 5DIII manage to do well at high ISO in terms of noise visible in the image when read noise is so high?
Its more than just "read" noise...that is why I tried to use the term "electronic noise". There are a whole variety of forms of noise in an electronic system, including read noise, but also dark current noise, thermal noise, differential noise (due to inconsistencies in the efficiencies of individual transistors for each photosite), etc. There are ways of mitigating each of these forms of noise. Canon sensors only seem to do a basic form of CDS, or correlated double-sampling, which mitigates a certain amount of dark current noise. Sony sensors, on the other hand, have a variety of ways of mitigating most types of electronic noise, which is why their sensors perform better.
As for high ISO, I am not sure of the exact mechanics, but I believe the black point rises as you increase ISO (thanks to Canon's use of a bias offset of 2048, signal actually starts at -2048 and rises from there). You effectively lose DR on both ends, shadows and highlights, as ISO is increased. By ISO 400 or so, it seems the black point is high enough that it starts above the electronic noise floor, meaning the only really significant form of noise left is photon shot noise (a matter of physics and therefor beyond our control.) Sony sensors, it seems, don't have a bias offset, so they are pretty much always starting from the same point (zero, effectively), and they lose DR on the highlight end of the scale as you increase ISO. Their shadow DR seems to be pretty static.
Anyway, back to the bind. My landscape, macro, and astrophotography could really benefit from lower noise and better shadow recovery. Particularly my landscape work, which I haven't done much of over the last 8 months, and none of whatsoever this year so far. I always find myself needing to push shadows around, and they have never really looked all that good, on any of the Canon cameras I've used (450D, 7D, 5D II). My options are either to add Nikon to my overall kit, deal with a menu system which irks me and body ergonomics that don't fit my hands right...deal with what I've got...or deal with what Canon has to offer. None of those options really solve my problem well, and the only one that solves it at all will also cost thousands of dollars for a new body (D800) and a couple lenses to cover the bases...such as a 14-24 and maybe a 24-70. Thats about $7000 worth of gear...something I can't afford at all right now (I can't even afford a 5D III, which I expected to list for around $3000, some $200 over budget already.)
Have you considered a D5100 or D7000? They'll get almost as much DR as a D800 for a fraction of the cost.
Well, for landscapes, I really want that extra resolution. A low resolution of 12mp really doesn't interest me much, and I was hoping the 5D III would land with 28-32mp. The D800E with 36.3mp and its incredible DR is about as close to the holy grail of landscape photography as I think it can get these days. I'd much prefer if Canon could reciprocate with their own megapixel/DR monster, though.