Bob Atkins from photo.net compared the Canon 5D Fullframe camera with Canon 20D APS-C 1.6x cropped camera. There’re possibly four different methods in the comparison and he adopted the fourth way. Here’s a real world image comparison. shot using test method (2), i.e. a zoom lens at different focal lengths on the EOS 5D and EOS 20D. This would probably be the practical situation if you had both a 20D and a 5D. You’d probably use the same zoom, but at different settings. I tried to equalize image quality by shooting at f11 and shooting at the wide end of the 70-300/4-5.6 IS USM, where image quality is high and doesn’t change a lot. In addition to comparing image quality in terms of resolution, you can compare EOS 5D vs. EOS 20D noise levels since these images were shot at ISO 3200. First, here’s the whole image:

“It’s a win, but not a total “slam dunk” for the full frame sensor, at least as far as the EOS 5D vs. EOS 20D goes. With good lenses the results from the EOS 5D are better, though with noticeably higher vignetting when lenses are used wide open. Of course you can mostly compensate for vignetting in software, but still you may lose something. With low cost lenses shot wide open, the full frame advantage disappears, and in situations where you have your longest lens in use on an APS-C camera, switching to a lower pixel density full frame camera and cropping will lead to lower resolution images.

However there’s no denying that under the right conditions, the images from a full frame camera like the Canon EOS 5D can beat those from an APS-C camera like the Canon EOS 20D, as the real world examples above show. Given the budget, I’d own both a 5D and a 20D and I’d use each in the situation for which it was best suited. Since I do a lot of nature work, the higher pixel density of the EOS 20D is useful for me. If I didn’t shoot extreme telephotos, I’d certainly pick the EOS 5D over the EOS 20D, especially if I had a good set of lenses to go with it. Good here doesn’t mean all “L” series either. It’s clear that the better “consumer” grade lenses can also take advantage of the higher performance of the full frame EOS 5D.

Canon have made a significant step in DSLR technology by producing a full frame camera which is selling for under $3000. They seem to be on a path which may end up - in maybe 3 to 5 years time - with a full frame “prosumer” DSLR sellng perhaps in the $1500 region. However currently, and for the forseeable future, the EOS 5D is second only to the $7100 Canon EOS 1Ds Mk II in terms of image quality and is, I think, the best bang for the buck out there if you’re looking for the highest image quality from a DSLR.”

Website : Full Frame versus APS-C sensors