I’ve been meaning to add my two cents to the discussions about sensor resolution that were going around recently. This much I know from doing some “engineering” resolution testing, for my Canon XTi and my two Canon L lens, 17-40mm f/4 and 70-200mm f/4, the very best (optimum) image resolution is at f/6 and f/7.1 respectfully.
FYI, “engineering” resolution testing is a very practical methodology; I photograph a complex carpet with a ton of texture using the XTi on a tripod and use a cable release for the exposure in the Av (Aperature priority) mode. This is versus the “scientific” methodology using optical charts under very controlled conditions.
The bottom line is that when I look at the test images at 200% and 300%, I have the most detail in the f/6 and f/7.1 exposures. When the aperature is just above my optimum, the image quality is barely different and by the time I get to f/9 I start to lose enough detail that the sharpening software doesn’t restore it. At f/16 and smaller, I lose a lot of contrast and detail that I have never been able to restore. Even at f/6 & 7.1, I still have to do a small amount of image sharpening to compensate for the the sensor filtration on the camera. But still side by side, f/6 and 7.1 is really amazing and fantastic when compared to an f/16 or f/22 image.
By the way, why evaluate my test images at 200 or 300%? Because I make 16×20″ printed images and I want to know what I may get if I use my 10.1 Mp file and do some moderate cropping and still make a 16×20″ print.
So now I keep my Av parameter set at f/7.1 (I can barely see the difference between f/6 & f/7.1 at 300% for the 17-40mm lens) to optimize image resolution. I can make informed changes to this “optimum” aperature knowing that I am potentially compromising in image quality, but there may be a compleing trade-off. Unless I want to create a soft focus image, I don’t plan to go beyond f/11.
Which means that if I want to consistently make images that need f/9 or f/11 (or greater), then I will need to change cameras/sensor systems, such a purchase a Canon 5D or the 1DsMark2. For the record, being an old school film guy, at first I did not believe that the choice of sensors will also determine your best lens aperature shooting process. But after reading the sensor reviews and then doing the testing, I now fully understand one of the trade-offs when going to a DSLR from my 6×6 SLR. ..and I still use the DSLR because what a great work flow and a wonderful new set of creative tools.
Best regards, Doug
Singular Images 
Doug, that’s and interesting experiment. I’ll have to try it myself with my D2X; however, it is very rare that I shoot below f/8. I’ve always been, since my film days, a wide open aperture kind of guy. I prefer shooting at maximum aperture or perhaps 2 stops down, at most. I just like really blurred backgrounds. The only exception that I make to this ‘rule’ is when I’m shooting with my 11-18mm lens, then I’ll shoot at about f/8. Hmmm, I wonder how it ‘specs out’ in the real world.
Thanks for the post and the idea!