So, I picked up a 5D Mark III the other day. I didn't quite plan on it, but I walked into my local Fry's and while they didn't have one on display (most retail stores don't yet), I curiously browsed the cages below the displays to see what they had in stock (as normally there will be things that just came in or haven't been inventoried yet.) To my surprise, there was a 5D3 sitting right there, and seeing as how they're a little difficult to get right now, I snatched it up. I often shoot indoor, low-light events, usually without flash, so I'm always looking for great high-ISO performance. Everything I've heard, read and seen about the 5D3 touts its great ISO performance, even in extreme conditions. Most of the videos i've seen show very usable files (for my purpose) up through its highest native ISO @ 25,600, and even at its H1 expansion ISO of 51,200 depending on the scene. At H2 (ISO 102,400) the files start to exhibit extreme luminance noise and it starts to look like surveillance footage. Probably more of a novelty than anything. But again, I wouldn't hesitate to use the 5D3 at ISO 25,600 or even 51,200 if the situation called for it.
In another un-scientific comparison, I decided to test the ISO performance of the new Canon 5D Mark III, 7D and 5D (original, "Mark I") at ISO 3200 where I tend to do a lot of my shooting. I will do a test at ISO 6400 as well. I did this mostly to compare noise at high ISOs between the 3 cameras.
The images were shot in JPEG Large/Fine on each camera, with a 35mm f/1.4L. Same exposure settings (1/60, f/2, ISO 3200, Auto WB) Shots were handheld (Disclaimer: again, "unscientific".) The comparison images from the two higher-resolution cameras (7D and 5D3) were scaled down to 12MP to match the resolution of the 5D (4368px on the long edge). No other processing has been done.
The upper right of each photo shows a 100% crop of roughly the same area from each photo.
Obviously the 7D missed focus (as 7D's tend to do,) but the point of the comparison was to compare high ISO noise performance, and it appears that the 6-7 year old 5D still fares well in this test against the newer 7D (DIGIC II vs Dual DIGIC4), due to its full frame 35mm sensor size. It's no surprise that the new 5D3 with its improved DIGIC V+ processor blows the other two away.
the 5D3 goes up to a native of ISO 25600, expandable to 50k (H1) and 102k (H2).
Here's the 5D3 at ISO 25,600 (click for full size):