UPDATE: I posted more thoughts on this subject and a reader’s comment Here.
So I just remembered tonight there was a full moon. I hurriedly pulled out my camera gear (it was already close to 10:30pm) and headed for the back yard. There were some nice clouds passing over the moon and I thought it should make for a great time lapse. Then wondered what camera should I use?? I remembered the 7D and my 70-200mm f/2.8 IS II gave me a spectacular image with the ease of a point and shoot. But that was over a year ago and now I have the 5D Mark-III. But then I thought it’s full frame sensor meant less coverage of the point in the sky (moon) I was trying to photograph. But it had a technically superior sensor and higher overall resolution. I thought this might be enough to match the high pixel density of the 7D.
I was wrong. This was one of the classic examples of a situation where the 7D is just a way better camera for the job hands down. Both shots were at f/4, ISO-100 and 1/1000 second. AF on and IS on (mode 1). At ISO-100, there was no advantage to the newer 5D Mark-III sensor. Both were right at home with the higher shutter speed. AF was spot on and on the 7D seemed a hair more confident/quicker than the 5D Mark III. Yes, I know the DIGIC5 processor is supposed to be 17x faster than the DIGIC 4 but as a software engineer I’m pretty sure having a processor dedicated to AF (7D has Dual DIGIC4 Processors) means parallel AF processing. In real life it’s either a wash or still a better performing configuration than a single DIGIC5.
This simple quick and rough test just confirmed my suspicions and what I’ve always tried to explain to some fellow photo geeks. Just because it’s a full frame camera does not necessarily mean it captures more detail in the true technical sense. The comparison here shows the 7D delivering the full moon with easily 2 times the number of pixels the 5D Mark-III and with similar clarity and sharpness using the same lens and settings.
I still maintain, for most other general scenarios and large field of view the 5D Mark-III will yield a superior image. However the 7D with it’s considerably higher pixel density will always yield much more detail (in this case, twice the detail). This is one of the many real life, real world scenarios that no ISO-Chart, spec sheet regurgitating blogging, rent-a-camera-for-a-weekend-reviewer will ever tell you. If you haven’t yet figured it out, I pretty much loathe most of the talking/blogging heads on the web just echoing whatever everyone else says.
This is one reason why I still hang onto my 7D. It is an amazing camera system. However it is imperative that a 7D owner only mount the best lens optics that money can buy because of the unforgiving and high pixel densities. The key is to know your shooting style, know and understand your camera system to make full use of it’s potential.
Yes I do own both camera systems and actively shoot both.
PhotoSnob 
Agree with all of the above.
There is nothing that can replace that 1.6x crop factor when you need the reach. That and the 7D will be using only the ‘sweet’ part of your 70-200mm f/2.8L IS II (although even on a FF the lens shows great edge-to-edge sharpness). I only rarely shoot sports, but if I did, I would definitely have a 7D (or pick up the rumored 7D mark II when it arrives).
Very good information. Thanks for sharing. 7D + good glass = happy campers 🙂
Thanks guys. Here are the simple numbers:
7D Sensor: 19,000,000P/333.27mmsq = 57010.8 Pixels / mm squared
5DIII Sensor: 23,400,000P/864mmsq = 27083.33 Pixels / mm squared
So yes there are 30,000 (2.1x) more pixels/photocells for every square millimeter of 7D sensor than there are on the 5DIII sensor. This means that at all focal lengths, compared to the 5D sensor, the 7D sensor is able to register 2.1x more image data for any given part of it’s cropped frame. Now of course, as I said, provided you are using a lens that is sharp enough. 30,000 more pixels for every square mm is a staggering number.
Yes, I fully realise the adventage of 7D over 5D3 in pixel counts.
Most of my wildlife shots are done with 7D mated with my 100-400mm L lens, handheld mostly.
The 5D3 is great though with my 24-70mm L lens.
So how is the 5d3 with 100-400 lens?
In my experience it did very well for image quality except the overal issues that I had with the 5D3 compared to 7D as described in other post.
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After 3 years of 7D use I was quite disappointed when tried the 5D Mark iii in the field settings: (1) poor light sensitivity for autofocusing: in the settings where the 7D gets the focus immediately, the 5D3 hunts forever; (2) slow autofocus on moving subjects (eg sport or wildlife); (3) at least my specific exemplar of 5D3 always tended to under-expose, both in the ambient light and with a flash; (4) with a flash, the 5D3 did 50% of under- and other 50% of over-exposures. So much unpleasant experience!