Whereas before you could get away with relatively large apertures on less-than-world-class glass, now you really do have to stop down to get to the good stuff.
This is the paradox. There's no doubt that f16 is not
the aperture at which 35mm lenses are sharpest. Most lenses are sharper at f8 and a few of the really expensive lenses are sharpest at f5.6 and even f4.
The system is only as good as its weakest link, and the sensor - which has for so long been the weakest link in high-end digital photograhy - is now not necessarily so.
Where's the evidence for this, considering that the pixel spacing of the 5D and earlier 1Ds cannot resolve greater resolution than that provided at f16 (now confirmed by BJL, our resident lens expert ). If the sensor is no longer the weakest link, I'd expect to see that increase in resolution between f4 and f8 with all good lenses. Instead, what I see is just a marginal increase in the contrast of micro detail at resolutions that are far below the resolution limits of the lenses at their sharpest apertures.
My impression is, to get just a marginal increase in image quality, we have to get a substantial increase in lens quality. The sensor is still the weakest link. An indication of this is the usual fall-off in resolution at the edges and particularly in the corners of full frame images.
If you've ever looked at the Photodo MTF charts for quite a number of Canon lenses they've tested, you'll see that resolution (or more accurately the contrast at specific resolutions) falls off dramatically usually in that part of the image outside the crop area of the APS-C size sensors. A disadvantage of moving up to full frame 35mm is this expectation that image degradation might be a problem towards the edges. It can
be, but my experience so far with the 5D is that resolution fall off towards the edges is not nearly as dramatic as indicated in the Photodo charts and less than my worst fears.
Finally, at the risk of antagonising BJL , I'll use the example of the Olympus 4/3rds format. The Zuiko lenses designed for this format are by all accounts superb. Far better it would appear than their Canon counterparts. Yet, if you check out the very detailed and thorough reviews of the Olympus 8MP E-300 and E-500 at dpreview, showing comparisons with the Canon 350D and 20D, you'll see that absolute resolution is a shade less than that of the Canon 8mp cameras.
But the Zuiko lenses are, I suggest, a lot more than a shade better. Of course you could argue that the 8MP Olympus sensors (manufactured by Kodak, I believe) are just not as good as Canon sensors. In the noise department, they don't appear to be. But resolution?
Unfortunately, there are no adapters for using Zuiko lenses on the Canon 350 and 20D, so we'll never really know what a really good lens can do when coupled with a really good sensor .