A lot more than you may think, I would say. And not significantly less than what you would get with a Nikon D800. Photographer Ron Martinsen got a brand new D800 and did a quick and interesting comparison with the EOS 5D Mark III (while reviewing the former). Read more after the jump (crops provided).
There is no discussion about the resolution capabilities of the D800: 36MP are a lot, really a lot of pixels, and there are situations where you need this. Period. However, as has been discussed here and on other sites, the resolution galore comes with some drawbacks, the most notable being a not-so-good high ISO performance (which is way better in the 5D Mark III). Mr. Martinsen confirms this finding when he writes that «in good light this thing [the D800] is impressive» and that he does not «feel that the ISO performance is worth a hoot after ISO 3200». This confirms what we already know about Nikon’s D800: exceptionally good resolution, below the average high ISO performance. However, what really surprised me (again), is the amount of details that the 22MP sensor of the 5D Mark III is able to render. I provide two crops below (courtesy of Mr. Martinsen, JPGs out of the cam), the first one shot with the D800 the second one with the 5D Mark III. Judge by yourself and then tell me if you are still thinking you are missing something because you do not have a 36MP sensor. Regarding the in-camera-processing:
Canon’s in-camera JPEG is certainly more satisfying to my eyes, and in this case gives me an image I could technically call done and do nothing else. With that said, it does add more in-camera sharpening and seems to expose better so it gives this image an unrealistic advantage over what is possible with the D800 image with similar processing (I’ll do that later in another review).
Here are the crops. Not only does the JPG from the 5D Mark III look better (better colors, in my opinion), but I can hardly spot more details in the D800 crop. What I am questioning is the following: Is a 36MP sensor a substantial advantage in real world shootings? Ok, this are JPGs, and only RAW can tell the whole story. But then again: do you prefer to have 36MP or a camera that performs very good in high ISO and is a rugged all-rounder camera (the 5D3)? :-)
There is also an EOS 5D Mark III review by Ron Martinsen, with lots of pictures and samples. Don’t miss it. From the conclusion:
I’m very pleased with the high ISO performance and features of the camera that allow me to get the shots that in the past would have been lost while I went searching for my flash. I’m feeling very comfortable shooting in manual and letting the ISO dictate the exposure, but I do find it tends to shoot one stop darker than I like with no exposure compensation. As a result I find myself dialing in the necessary ISO as needed.
I feel totally comfortable with using shots up to ISO 25,600 with anything at or below 6400 being as solid as ISO 1600 and below on my 5D Mark II.