The "serious" equipment testers, the ones with the biggest web sites and the best methods, have all noted one thing: the absolute BEST lens resolution figures have all come from full-frame cameras. We need to keep in mind, that real-world optical performance is a measure of both the lens, and the capture medium used behind that lens. The vast,vast majority of lenses have some imaging weaknesses and flaws. A full-frame d-slr from Canon, Nikon, or Sony is one of only a relative handful of models, ALL of which were designed and built to provide the best image quality possible at the price point for the camera. Canon has had the 1Ds series and the two 5D models; Nikon the D3 (D3,D3x,D3x) series and the D700 and NO others; Sony has made the A900 and A850. I think, in total, that equates to 11 individual full-frame d-slr cameras. Eleven. Few enough models that the "concept" of a full-frame camera is much,much,much more-discretely definable than some vague, mythologized "thing". We are talking about 1) rugged, Abrams-tank-like Canon 1Ds bodies and Nikon D3 bodies; affordable and reasonably lightweight, unobtrusive Canon 5D and 5D-II bodies, two very solid workmanlike and VERY "analog-camera-like" Sony bodies with huge viewfinders; and the Nikon D700--as sort of low-priced, almost-flagship-Nikon body that has most of what the $5,000 Nikon D3 had, but at half the price.
The APS-C crop-body field is FILLED with models, priced from $499 to, well, I don't know...so many models it's hard to keep track of them...
But the one constant that keeps coming back is that the eleven full-frame cameras have offered what was, at introduction, state of the art resolving power, image size, dynamic range, and overall picture quality that was or is, as high as their makers could manage. Period. Canon's original 11 MP 1Ds was not that good compared to the later cameras, but it was high-resolution. Kodak's 14n and the SLR/C and SLR/N version cameras were made in such small numbers I did not include them in my original "Eleven".
Full-frame is an entirely different image capture medium than APS-C is. Pure and simply. It is a much bigger capture format. It works very differently with all lens lengths. An 85mm or 105mm lens is almost useless indoors in a normal house on a crop-body camera. On a FF camera, both are very valuable tools. In small studios, crop-body cameras force the photographer to use very short focal length lenses in order top encompass full figure, standing people, or groups, and with the small capture format, that means wide-angle views BEHIND the subject distance, problems with background width, backgrounds that are distressingly in-focus, and apparent perspective distortion on extended limbs, and on the size of people in the front row of group shots, as contrasted with people in subsequent rows or at different distances.
Here's a simple rule: The smaller the capture format is, the deeper the depth of field at EQUIVALENT angles of view. If you want to shoot pictures where the "normal lens" has relatively shallow depth of field,and tend to blow the background out of focus easily, then use a medium format size capture, like 6x6 or 6x7 film. If you want to shoot pictures where the normal lens has pretty good depth of field, APS-C is nice. If you want to almost never have to worry about focusing, shoot with a camera that has a very SMALL capture size sensor or film.
