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I enjoy reading things that make my head hurt, it usually means I’m learning something. Roger at LensRentals.com is always good at it, I’m now going to have to pay attention to Alex Cooke at F-Stoppers. He has written a great article about diffraction the how it pertains to the resolution of the upcoming Canon EOS 5DS and EOS 5DS R.
On a 5D Mark III, the size of the Airy Disk begins to exceed the size of the circle of confusion just after f/11. This means the 5D Mark III reaches its diffraction limit at that point, the point at which diffraction begins to become visible when viewing an image at 100% at a typical viewing distance. This is different from the diffraction cutoff frequency, the point at which airy disks completely merge and no amount of stopping down will improve resolution. Think of the space between the diffraction limit and the cutoff frequency as the space of diminishing returns. On the other hand, the 5DS reaches its diffraction limit just before f/8, slightly over a full stop sooner than the 5D Mark III. This might have landscape photographers and those who rely on having a large depth of field worried.