Quote: One of the formulas in the film days was 1380/f # to get a general idea of the maximum LP/mm due to diffraction. I do lots of macro work at f/45 with a 6x7 and am not really concerned about the slight loss due to diffraction.
Your formula is f-stop vs line-pairs, which doesn't say anything about lens or film size. In other words, diffraction is not about lens size or camera format, only about aperture and pixel size.
You win big on a 6x7 camera because you have film that is more than 4.5x larger than 35mm! You scan it at 3000 dpi (56 mpixels equivalent) and print natively at 22x28 inches at 300 dpi. A 3000 dpi scan is about the same as 8.5 micron dots, which is like the Nikon D700, which has a 12 mpix, full frame sensor. Drop to a high-resolution crop-frame sensor, like the K5 and D800. These both have 4.8 micron pixels, and produce an 22x31 inch print at 150 dpi. No wonder diffraction doesn't show up in your 6x7 prints!
Digicams have very small pixels (under 2 microns); they show pixel-level loss of contrast due to diffraction by or above f/4. Pentax K5 and Nikon D800 both have 5.8 micron pixels, which may start to show diffraction by f/5.6.
The old rule of thumb for 35mm was f/8 and be there, which gave you a great, real-world balance between depth of field and diffraction. Pixel-peeping on today's small pixel cameras lets you see where you want to set your rule of thumb. Do a slideshow of a detailed image shot across all your apertures to see how depth of field and diffraction trade off with each other (Or subscribe to Diglloyd).
Pixel-level camera shake and mis-focus show up on these new, high-res cameras, also!