It depends a lot on your expectations, your definition of an "issue" and what back you have.
Diffraction is usually defined by: r = 1.22*Lambda*N, where r is the radius of the Airy circle, Lambda is the wavelength of light and N is the f-number, like 16 for f/16.
So, for green light around 0.5 microns and f/16 we would get 9.76 microns. But this is the radius, the diameter would be twice that, namely 19.5 microns.
Recall that the Airy ring is a circle, so it's area would be r^2 * pi, that is 299 microns.
If pixel size is 9 microns, typical of many MFDBs around 20 MP the Airy ring would cover about 3.5 pixels. O
n the IQ180 with 5.17 micron pixel pitch it would cover 11 pixels. Both sensors would loose some sharpness, but the 9 micron pixel sensor may loose little and the 5.17 micron sensor a lot.
So if you are very careful about having maximum sharpness and in utilizing your lenses and have a high resolving sensor like P45, P65+, IQ180, Pentax 645D or a Leica S2 you would consider diffraction. If you don't really care about maximum achievable sharpness it may be less of an issue.
This page illustrates diffraction on a 4.7 micron sensor: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1
. That would be pretty close to what you would see on an IQ180.
That said, diffraction is relatively benign to sharpening, as demonstrated here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=2
This is a good link for using the hyperfocal distance correctly: http://optechsdigital.com/Alpa_and_Hyperfocal.html
the article also demonstrates diffraction effects.
These two recent threads here on LuLa forums discuss this issue: http://www.luminous-landscape.com/forum/index.php?topic=59862.0http://www.luminous-landscape.com/forum/index.php?topic=59930.msg483360
What is the best way to test a lens for diffraction?
Is it true that medium format lenses can go to much higher f-stops than 35mm DSLR lenses before diffraction is an issue?
I am asking this because I would like to avoid having to use focus stacking in my landscape images.
In the Luminous Landscape video series “Camera to Print & Screen” Michael Reichmann says: “With a pocket camera you may start to hit diffraction at f/8, with a DSLRs it is usually about f/16, but on a large or medium format camera you might be able to go to f/32 before you hit diffraction.”
Jeff Schewe: “Maybe, but that is something you have to test with that lens and sensor combination.”
Note: This is an approximate quote from about 4 minutes in to chapter 3 of “Camera to Print & Screen”