Okay, since we still have so many threads on 7D comparisons to other bodies, I figured since I did a high ISO comparison of my old 50D to the 7D last year before I sold the 50D, I would do another comparison of the 7D, this time to the 40D I am about to sell. (maybe I should keep it?! 
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This time, I didn't concentrate on high ISO so much. Instead, I tried to pick a scene that had fairly high DR with lots of colors. I went both ways with each comparison, I resized the 7D down to the 40D, and resized the 40D up to the 7D.
You can see all photos (original sizes accessible) in my 7D vs 40D Gallery
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Here are the conditions of the IQ test:
- ISO 400
- f5.6
- AWB
- High ISO noise reduction off on both
- Evaluative metering
- 17-55 IS lens
- Same picture style settings
- Post processing in CS3, autolevel and USM, same values in all cases (even though the 7D requires a bit more than the 40D to get the most IQ)
I then set the ISO to 3200 and shot a colorful scene in low light so you could compare both.
My impressions are that the 7D does wonderfully well consider the large boost in resolution, and even though you could resize your 40D images to the 7D resolution, details obviously get muddied. ISO performance at 3200 is also better, and the gap grows past ISO 3200. The 40D is no slouch though either and really, as stated in other threads, you buy the 7D more for its feature set (video, wireless flash, better LCD, larger vf), AF improvements, weather sealing, and high ISO performance more than IQ. That being said, the IQ is better with the 7D, and the colors seem more realistic, so you win there too.
In any case, these are my opinions. I will post a few samples below, and you can review them here, or look in the gallery for all the pixel peeping glory you want!
Here is the general scene, as seen by the 40D and 7D sensors, then some comparisons below that.
ISO 3200 crops, first 40D vs 7D resized to 40D, then 7D vs 40D sized to 7D
I hope you enjoy this, it should create some interesting debate!
