As a craft, photography is a borderline miracle....................

But the process that governs this miracle is bounded on every side by constraints.
Some minimum amount of light is needed.
Some maximum amount of movement is tolerable.
A maximum distance to the subject must be managed.
A maximum amount of haze in the air is tolerable.
I can't shoot through heavy rain.
I can't shoot through blowing dust or sand.
Wrong color light messes up my pictures.
It's hard to make a perfect lens.

All of these constraints must be balanced against each other when taking a picture. Many situations, of course, are set so deep in the middle of everything that the constraints are far, far away. But when one is working on the edge, things can be much different. ....................

I’ll just opine that lots of the commenters that ignore diffraction are the first to advocate spending max $ or “good glass” to get that last 10% in sharpness and contrast, only to lose it shooting at f22 instead of f11 when as Bill B demonstrated, a DOF calculator would show that often the small increase in DOF is not necessary or noticeable.

If you’re going to spend 2-5x the cost of a mediocre lens, why not get the best out of it? It’s easy to say that 10% isn’t noticeable, but then there’s the 10% higher noise from the 4x higher ISO required and another 5-10% from ignoring other basic principals, and in the end, you end up with a mediocre instead of a tack sharp image and then justify it by stating that “there’s too much emphasis on sharpness” or “quit worrying about the details, photography should be fun” or “looks good enough to me - you can’t see the difference”.

Producing first class work/perfection, whether it’s photography, music, art, woodworking, or any sport, is all about attention to detail, and details add up.

There are easy two experiments you can perform to 'prove' diffraction:

1, Find a landscape situation, even just shooting down your street in the neighborhood. Set up your camera on a tripod and shoot in aperture priority mode. Compose an acceptable image, with focus set on something either 1/3rd into the frame or say a mid-point between foreground and background of the scene. Walk the aperture of the selected lens from wide-open through the smallest aperture. 1 or 2 frames per every aperture with no other changes. For extra credit, adjust the camera one-quarter turn so one of the corners of the frame passes through the horizon so you have 'details' in the corner.

2, Use a macro lens and select a detailed close-up subject, such as a flower or coin, similar. Again, set the camera on a tripod, walk the aperture from wide to smallest in aperture priority.

Take all these digital files and zoom to the 1:1 pixel-level details. Walk the images reviewing the same spot in every image at the 1:1 level of details. In the landscape, look at the far-off details in the 'upper corner'. Watch how, as the aperture gets smaller from wide open, first the details improve, then reach a maximum sharpness, and then get worse. As they get worse, you're seeing lens diffraction. Note the aperture where things get worse from the max / best sharpness in the distance corners.

The same macro / close-up digital images will do the same thing. Look at the specific point of focus and slightly around that point. Again, watch how things get better, stay sharp, and then begin to get worse as the aperture gets smaller and smaller.

Diffraction is not some urban legend off the internet. You can see for yourself, from the charts above or your own demonstration images. The urban legend is actually the false benefit of small apertures in digital photography, passed around as 'useful tips' by the likes of ancient classics like Understanding Exposure and half the UHH community.