Diffraction Dose it Matter

I’m no expert on Diffraction and don’t really understand the physics of it, I believe its like a hosepipe when you force water through a small hole it tends to spread out. When light rays travel through our lens using a small aperture it has a similar effect. This reduces the quality of the image, apparently the bigger the pixel size the less effect diffraction has on your image quality, so on your APS-C (400D)10mpix sensor will suffer more than your (5D)full frame 12mpix.
I tend to avoid very small apertures unless I really need that massive depth of field generally when shooting with wide lens your apparent DOF is greater at fairly modest apertures with longer focal lengths the need to stop down is greater but I personally tend not to need to be so close to my foreground when compressing the landscape. My main reason for trying to stick to above F16 is to avoid having to clean dust spots of my images(not such a problem since I got sensor brush), I know I see a lot of landscape images were the photographers seems to use F22 as there standard setting but dose diffraction actually make that much difference.
I tested using 3 different lenses a sigma 28mm F1.8 prime, canon 17-40 F4L and canon 24-104F4L. I shot a serious of images F22,F18,F14,F10,F4. The camera (400D) was mounted on a tripod with mirror lock up cable release and manual focus. I viewed these at a rather extreme 200% in DFP it was quite obvious that the quality suffered at the extremes of aperture as you would expect wide open F4 on the canons there was an obvious difference from F10 also the difference in all cases between F10 and F18 was noticeable becoming more pronounced by F22. The deterioration at the large apertures(F4) is obviously to do with the optics.
F22
F18
F10
I don’t know if the deterioration at F22 is optics or diffraction but it is there in all 3 lenses the fact that I know this is all that matters,
I suppose for us as photographers dose it matter, for my landscape work who knows when a gallery/customer may want a very large print I always try and get the best quality out of my equipment I’m sure at A3 you would not be able to tell the difference, but at A1 or A0 it might be a different story.
It would have been very interesting to do the same test with a 5D and see if there was much difference I suppose this would have proved to an extent if the deterioration was optics or diffraction unfortunately I don’t have access to one so maybe somebody else could take up the challenge.
At the end of the day I will use what ever aperture I need to keep the important elements of the image in sharp focus but I think its worth thinking about do I really need to use F18/F22 for the image to work.
An interesting site with a technical explanation

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