Actually, to be strictly true, diffraction depends on the size of the aperture in relation to the focal length from the aperture to the image, whether there is a lens or not and is related to the circle of confusion.
Whether it is relevant to an image or not depends on the enlargement factor and viewing distance from the printed image.
I did this series of images to demonstrate diffraction vs depth of field for a course at our photo club…
More precisely, diffraction depends on the angle of the light cone formed by the aperture diameter and the distance to the sensor. The smaller that angle is, the more diffraction is visible. There is no relation with the circle of confusion. Diffraction is mostly visible when the coc is 0 or just a minimal point, theoretical super focused. The bigger the coc the less diffraction is visible.
In your picture you probably focused on the body. When making the aperture smaller, then the background is getting within the dof limits.
If you want to make diffraction visible, you should make a picture of a 2 dimensional object.
George
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I focused on the strings. After more than 50 years of experience in photography, I do know what I am talking about.
You didn’t mention you focused on the strings. One reason to use a 2 dimensional subject for that lesson.
George
Joanna, that would be ideal but your 50 years of experience is going to take me to over a 106! 
I’m a calligrapher by trade and I doubt I’ll ever get past that amateurish dependence on the digital assistance modern cameras give me. I can say, however that I am resorting to bracketing and exposure compensation less often. And I do realise that the “blinkies” (new word for me there!) reflect the cooked preview. I’ve kind of gotten to know how much I can trust it. I love the dynamic range of the D750!
Yes ofcoarse. But the showing of diffraction is a combination of sensor resolution and lens properties. Higher resolution shows airy disk earlier.
I can’t calculate it but a g80 16Mp and a certain lens or a G9 20Mp with the same lens could cause a stop less of f-number because of the different size of 1 pixel.
The smaller the pixel the earlier the surface is covered by the blur.
ISO is mostly sensor(size and resolution) and ADC properties and characteristics which makes the noise vs signal threshold of not acceptable. (which is for everyone his own choise.)
But the point i like to make is even non proffesional photographers can create with there equipment better images if you take the time to read about theory, geartests, ask around when needed to grasp the info, and then just test this with there camera setup.
Like exposure vs electronic exposure, (iso and post correction)
Or trying to gues the focus point made by AF. A point does 1/3 infront and 2/3 at the rear of DoF. Things like photographers app on your phone help alot to visualise action and reaction. DoF and focal distance relation.
The more you remember the better you can make decisions to get the best out of the situation. No matter the camera. (i see often people with apsc and a superzoomlens on full auto AI clicking around, which isn’t wrong but a small effort can make such a differents in enjoying the hobby and you get better images for free.)
