>>3273121You're missing something very important.
Diffraction mostly depends on 2 factors:
1. The physical opening of your lens aperture. The larger the opening, the less the diffraction.
2. The distance the light has to travel until it hits the film, after passing through the aperture. The longer the distance, the more diffraction you get.
You're missing the second factor. The best approximation, in most non specialised designs, of the distance the light has to travel after passing through the aperture is the focal length of the lens. (Assumptions being that the lens is roughly symmetrical and the aperture roughly at its optical centre, which is most often the case, or close enough).
So, diffraction is proportional to physical aperture opening, and inversely proportional to focal length. We combine those 2 to say diffraction is proportional to f/number. That's all there is to it.
So again, it's the *ratio* that matters, not the physical opening. That defines the boundaries of the airy maxima and minima.
Seeing your graph is from wikipedia, I know why you might have gotten confused. Wikipedia gives a formula for the *angular* separation of 2 objects. But the angles of those 2 objects depends on focal length (wideangles expand perspective while teles compress it). So the dependence on focal length is almost hidden in the the angle variable (θ).
However, just a couple lines further down, you can see how you can convert the angle between objects to distance between objects *on the film*, which is what matters (all measurements of diffraction and resolution are in lp/mm, which is distance between the lines *on the film*, not the angles that the lines form physically when viewed on the target).
Then you sinθ becomes x/f, so pic related makes it clear that it's the f/number that defines diffraction.
>I typed all that shit to correct some misconceptions (diffraction depends on aperture size only, diffraction depends on pixel size, etc), not to win an argument. 
