I don't know if that holds water anymore. The modern LF lens is darn good today. I'm seeing resolutions of 60-80 LP-MM with the newest generation and the resolution holds up quite wel, nearly to the edge of the image circle in the Super Symmars.
The reduction in image magnification is what makes the LF film camera interesting. Much greater apparent DOF before defraction kicks in.
I disagree on both points: smaller format lenses maintain a very big resolution advantage over even the latest lenses for 4"x5" format, measured by MTF at equal lp/mm, and the trade-offs possible between DOF and diffraction are the same for any format. Only the use of view camera motions given any advantage to a larger format on apparent DOF when diffraction effects are equal, and view camera motions are coming to 35mm format digital now.
I) The Super-Symars you cite are still not even close to good 35mm format lenses for "absolute resolution", MTF at equal lp/mm. (They probably are sharper in line pairs per pictuer height, and thus for lp/mm on prints of equal size.)
Compare the MTF curves for two normal primes, the Super-Symmar XL 210mm f/5.6 at
and the Canon 50/1.8 II athttp://www.usa.canon.com/consumer/controll...52&modelid=7306
I see the Canon having better MTF at 30lp/mm than the Super-Symar at the lower 20lp/mm, and the Canon at 10lp/mm is way ahead of the Super-Symar at 10lp/mm or even at 5lp/mm. And that is comparing to a very inexpensive non-"L" lens! Comparing instead to the Canon 50/1.4 (still not an "L" lens, but pricier) gives similar results.
Going to an even smaller format, the inexpensive Olympus 4/3 format 35mm f/3.5 macro lens http://www.olympusamerica.com/e1/sys_lens_35mm.asp
does even better fr lp/mm of resolution. Its MTF at 60lp/mm and wide open f/3.5 is far better than the Super-Symmr at 20lp/mm and any aperture, and the 35/3.5 has MTF at 20lp/mm as good or better than the Super-Symar at even 5lp/mm.
Even a mid-priced small format 4x zoom lens like the Olympus 14-54/2.8-3.5http://www.olympusamerica.com/e1/sys_lens_14mm.asp
has better MTF at 60lp/mm (wide open or f/8) than the Super-Symar at 20lp/mm, and the 14-54 at 20lp/mm trounces the Super-Symar at 20lp/mm and 10lp/mm, and outdoes even the 5lp/mm Super-Symar performance.
At the risk of being "too theoretical", any resolution improvement in a larger format can carry over to a smaller format, and smaller format makers like Canon have a lot more money and resources, so are certainly capable of making such improvements if they choose. Downsizing a LF lens design in all dimensions by a factor of four shrinks the focal length by the right factor to give the same angular FOV, and shrinks the size of all "design" abberrations by the same factor, essentially giving four times the resolution (same MTF ast fout times the lp/mm). This downsizing gives equal aperture ratio, as was used in the tests (f/11 for all). Arguably (see below) the smaller format woud mor often use a lower apertrure ratio, worsenign abberrations.
II) On the second point about DOF and diffraction: the possible balances of DOF and diffraction effects on print sharpness are independent of format, once you level the playing field by viewing prints of the same size from the same distance. In fact, they are each determined solely by the effective aperture diameter (focal length divided by aperture ratio), so you get the same DOF and diffraction results in different formats by adjusting aperture ratio in proportion to focal length.
Why? Famously, if a larger format and longer focal length are used with the same aperture ratio, the larger format gives less DOF. In detail, the sizes of the circles of confusion on the film or sensor increase with the square of focal length, but then degree of enlargement is reduced in proportion to focal length, so the CoC sizes on prints are larger in proportion to focal length.
To get equal perceived DOF, this must be compensated for by increasing the aperture ratio in proportion to the focal length (that is, using the same effective aperture diameter).
Then the diffraction spot size on the film/sensor is determined by aperture ratio alone, independent of focal length, so once you choose apertures to get equal DOF, the diffraction spot size on film/sensor is larger in proportion to focal length.
The smaller degree of enlargement needed with the larger format just cancels this out, so the diffraction spot sizes (and circles of confusion) are the same size on same sized prints, from any format.
P. S. Large format enthusiasts should probably stick mostly to the legitimate claims of
1) greater print resolution in lp/mm on equal sized prints, after the lower degree of enlargment needed
2) finer tonal gradations/greater dynamic range
to explain this preference. Those two have always been and probably always will be the main real reasons to sometimes choose a larger format than most others photographers use.