If I have understood Prof. Johnson's article, he is suggesting f21 as the aperture for maximum DOF with FF sensors while maintaining adequate suppression od diffraction effects. My best recollection is that this is the highest f-number that I have seen recommended. Other writers are suggesting values betweem f11-f16 for FF. How do we evaluate his assertion? Put another way, I am seeking max DOF with sufficient IQ for a 16x24 in print with my 5D, what apertuure should I choose?
There will come a point, as you stop down, when the increase in blur at the precise point of focus will equal the increased sharpness at the extremes of the DoF range (in the close foreground for example). At such a point you will have maximum DoF consistent with 'good' resolution. It then becomes a personal choice whether to trade off maximum DoF for a subtle increase in sharpness at the point of focus, and that choice may also be influenced by lens quality.
For example, taking 2 shots of the same scene with my 24-105 zoom and 5D, at f8 and f16, I would expect to find that the point of focus in the f8 shot would be very marginally sharper than in the f16 shot (but nothing to worry about). However, the extremes of the DoF range, in front of and behind the point of focus, would be significantly
sharper in the f16 shot.
If I were to use a high quality prime, I would expect the plane of focus in the f8 shot to be slightly better than 'very marginally sharper'. As one stops down into the region dominated by diffraction, all lenses tend to be equal.
Generally, I find when maximum DoF is desired, using my 5D at f16 will deliver the goods without any significant sacrifice of resolution at the plane of focus, but clearly this is not a 'cut and dried' situation. After all, people pay a lot for high quality, razor sharp primes and understandably such lenses are not at their sharpest at f16.