Frankly I'm not so sure that actually applies to macros where the small details which would normally be affected by diffraction limiting are very large and so far less likely to be affected.
Diffraction limiting is associated with the airy disc but where, say, the eye of a fly is so tiny that at normal resolutions both it and the small details associated with it would be blurred, at macro sizes we can see the individual lenses in the fly's eye simply because, on the sensor, they are large enough to cover many pixels and thus be far less susceptible to diffraction limiting.
I don't understand your argument, but never mind about that. But is the implication that diffraction does not bring lenses to a common (low) level of sharpness/detail/resolution as aperture decreases, and therefore that better quality lenses do in fact have more resolution than lesser quality lenses even at small apertures - that is, that the quoted extract from the Joseph James article is incorrect (along FWIW with the results of my practical experiments)?
Edit: Re-reading what you wrote and thinking about it some more, I think I misunderstood what you wrote. I think perhaps you are saying that
at the macro scale diffraction does not occur, or does not occur so much as at larger scale, or occurs as much but doesn't have as much effect as at larger scale. Were that the case, at the macro scale good quality optics might still deliver more resolution than lesser quality optics at small apertures. Am I understanding you correctly?
Interestingly, although I don't understand the technicalities about Airy Disks etc, I can see that the Joseph James article doesn't qualify its discussion in terms of the scale of the subject/scene. For example, here is an extract from the article leading up to the bullet point that I quoted above.
In terms of cross-format comparisons, all systems suffer the same from diffraction softening at the same DOF. This does not mean that all systems resolve the same detail at the same DOF, as diffraction softening is but one of many sources of blur (lens aberrations, motion blur, large pixels, etc.). However, the more we stop down (the deeper the DOF), diffraction increasingly becomes the dominant source of blur. By the time we reach the equivalent of f/32 on FF (f/22 on APS-C, f/16 on mFT and 4/3), the differences in resolution between systems, regardless of the lens or pixel count, is trivial.
For example, consider the Canon 100 / 2.8L IS macro on a 5D2 (21 MP FF) vs the Olympus 14-42 / 3.5-5.6 kit lens on an L10 (10 MP 4/3)
. Note that the macro lens on FF resolves significantly more (to put it mildly) at the lenses' respective optimal apertures, due to the macro lens being sharper, the FF DSLR having significantly more pixels, and the enlargement factor being half as much for FF vs 4/3. However, as we stop down past the peak aperture, all those advantages are asymptotically eaten away by diffraction, and by the time we get to f/32 on FF and f/16 on 4/3, the systems resolve almost the same.
For the same color and f-ratio, the Airy Disk will have the same diameter, but span a smaller portion of a larger sensor than a smaller sensor, thus resulting in less diffraction softening in the final photo. On the other hand, for the same color and DOF, the Airy Disk spans the same proportion of all sensors, and thus the effect of diffraction softening is the same for all systems at the same DOF.
Let's work an example using green light (λ = 530 nm = 0.00053mm). The diameter of the Airy Disk at f/8 is 2.44 · 0.00053mm·8 = 0.0103mm, and the diameter of the Airy Disk at f/4 is half as much -- 0.0052mm. For FF, the diameter of the Airy Disk represents 0.0103mm / 43.3mm = 0.024% of the sensor diagonal at f/8 and 0.005mm / 21.6mm = 0.012% of the diagonal at f/4. For mFT (4/3), the diameter of the Airy Disk represents 0.0103mm / 21.6mm = 0.048% at f/8 and 0.005mm / 21.6mm = 0.024% at f/4.
Thus, at the same f-ratio, we can see that the diameter of the Airy Disk represents half the proportion of a FF sensor as mFT (4/3), but at the same DOF, the diameter of the Airy Disk represents the same proportion of the sensor. In other words,
all systems will suffer the same amount of diffraction softening at the same DOF and display dimensions. However, the system that began with more resolution will always retain more resolution, but that resolution advantage will asymptotically vanish as the DOF deepens. In absolute terms, the earliest we will notice the effects of diffraction softening is when the diameter of the Airy Disk exceeds that of a pixel (two pixels for a Bayer CFA), but, depending on how large the photo is displayed, we may not notice until the diameter of the Airy Disk is much larger.
Typically, the effects of diffraction softening do not even begin to become apparent until f/11 on FF (f/7.1 on APS-C and f/5.6 on mFT -- 4/3), and start to become strong by f/22 on FF (f/14 on APS-C and f/11 on mFT -- 4/3). By f/32 on FF (f/22 on APS-C, f/16 on mFT -- 4/3) the effects of diffraction softening are so strong that there is little difference in resolution between systems, regardless of the lens, sensor size, or pixel count.
We can now summarize the effects of diffraction softening as follows:
- Diffraction is always present. As the lens is stopped down, optical aberrations lessen and diffraction softening increases.
- The "diffraction limited aperture" is the f-ratio where the effects of diffraction softening overcome the lessening lens aberrations, and will vary from lens to lens as well as where in the frame we are looking (e.g. center vs edges, where the edges typically, but not always, lag around a stop behind the center).
- All else equal, more pixels will always resolve more detail, regardless of other sources of blur, including diffraction.
- All systems suffer the same diffraction softening at the same DOF, but do not necessarily resolve the same detail at the same DOF, as diffraction softening is merely one of many forms of blur (e.g. lens aberrations, motion blur, large pixels, etc.).
- As the DOF deepens, all systems asymptotically lose detail, and by f/32 on FF (f/22 on APS-C, f/16 on mFT -- 4/3), the differences in resolution between systems is trivial, regardless of the lens, sensor size, or pixel count.
And as it happens, the practical experiments I did, at macro scale, were consistent with the final bullet point.