All optical systems produce a blurred image as a result of diffraction. On a fundamental level, we require a ruler to measure how much blur has occurred in a system. MTF, MTF50, and other measures are all "resolved" quantities mathematically. They are produced by taking an intensity profile and performing some mathematics on it. These methods cannot tell the "source" of blur, only that blur has occurred.

When you consider things like chromatic aberration, however, it becomes clear that things carry some dependency on wavelength, or color. As it turns out this is also true for the wave behavior of light. Blue light doesn't travel faster than red light, but it carries more energy per photon and consequently blurs less when it diverts its path around an aperture. (E=mc^2 after all, it must be more massive and thus have greater inertial).

In this respect, we use the wavelength of light as a ruler. More massive photons don't divert as much, so they stay tightly packed and produce a small, high intensity spot.

However, this is largely irrelevant to photography, as consumer lenses are simply too aberrated.

Here I present to you the spots from several lenses, as examined on an MTF bench. The lens is f/2.4 and covers a 120 degree field of view. As designed, it is diffraction limited (corrected to less than lambda/6 waves of aberration, lambda/4 is generally considered diffraction limited).

First we have an excellent spot:

We may also see a disturbed optical system, i.e. one with some amount of misalignment. This particular sample has about a wave or so of coma on-axis. This is more typical of a consumer lens, since they simply do not cost enough to be designed and aligned to this specification (nor is it truly necessary).

As another example, here's about a half wave of coma, but also about a wave of astigmatism. Not very pretty.

Here's the MTF of the three spots in the same order:

Now let's look at a consumer lens universally regarded as being super duper sharp and a lens that is a favorite of mine, the Zeiss 100mm f/2 Makro Planar.

I apologize for the change of format. The big kicker here is that nowhere in the field of view does the MTF at 50lp/mm surpass that of the highly disturbed sample. It's at about 0.6 across the entire field, where the highly disturbed but perfectly designed lens achieves about 0.7 even in its worse plane.

Maybe in 10-25 years when consumer interchangeable lenses are designed as well as this $25,000 wide angle fixed lens will the airy disk matter in photography, but today it does not.