This means essentially that you cannot stop down to much if you want to have optimum sharpness. All lenses go diffraction-limited sooner or later. A perfect lens has it's optimum sharpness fully open. Normal lenses need to be stopped down a couple of stops to reduce uncorrected aberrations, around f/8 or f/11 they would also get diffraction limited.
What this also means that there is a maximum "sharpness" you can achieve at a certain aperture, and that amount is sharpness increases with aperture. Any fairly decent lens should be pretty good at around f/11, but at that aperture it could not utilize all the resolution in a 39 MPix, back. So if you want the best resolution you can get you would need lenses which achieve maximum sharpness around f/5.6 or f/8 and avoid stopping down to more than that.
Putting it another way:
1) If you are using f/11 or f/16 on a 39 MP back you could probably get the same sharpness/resolution/MTF from a 25 MP back.
2) To utlize a 39 MP back you need lenses having optimum performance around f/8 or f/5.6, and not stop down any more that.
3) You see why "scheimpflug" is such a good idea :-)
4) This is nothing really new, many of Canon's top (L-class) telephoto lenses perform optimally at fully open aperture.
Michael has done it again
An up to date succinct comment on where we have reached in great MF/LF lenses for digital backs.
Who else actually has the 39MP Phase back and the Rodenstock lenses to explore for us.
I learned more about "diffraction-limited lenses" and that the HT lenses get into this rare group.
The next thing I want to know is how this in fact changes resolution?
I'm particularly interested in the wide angle lenses and would love to see further derivatives of this kind of very practical review on LL.