It was pointed out again, and again, and again, and again, and again..........
I just still don't understand what it even means though. The depth of field changes for each aperture. How can one aperture be "sharper" than another? At f/1.4, my background isn't sharp. What exactly are we talking about anyways? I can understand a portrait shot, say at f/1.8, vs. f/2.8. Is that what we mean? The face is sharper at 2.8? Shooting at the same focal plane? On the other hand, at f/11, my whole scene is sharp. It just doesn't make a whole lot of practical sense to me, unless we're talking a flat surface focal plane, at the same distance.
Assuming a "perfect" lens (one in which zero optical aberrations are present...that is CA, spherical aberration, field curvature, etc.), the only thing that limits resolution (which is the objective measure of acutance and microcontrast) is diffraction. As your aperture gets wider, the effects of diffraction become less and less. Sharpness (in the context I believe you are using) is a somewhat subjective measure of acutance and microcontrast...how harsh edges appear, how fine detail is discerned...and as such sharpness improves as resolution improves. Higher resolution also allows you to resolve finer details, and resolve larger details more sharply, vs. only being able to resolve larger details. At f/22, your limited, by diffraction, to only 30 lp/mm. If the detail you are interested in is larger than 30 lp/mm, your lens will resolve them, and the larger the detail the more clearly it will be resolved. At f/8 resolution jumps to 85 lp/mm, allowing you to resolve detail less than half the size as what you could resolve at f/22. By f/2.8, resolution increases to 245 lp/mm...again allowing you to resolve more fine detail, nearly 3x more detail than you could resolve at f/8.
In the case of, say, a macro subject (such as an insect), or a bird, or furry animal...there is PLENTY of very fine detail. If we use birds as an example, every feather has a shaft, on either side of which is a vane, each vane of which contains barbs, and each barb of which contains interlocking barbules, each barbule containing multiple fine hooks that assist in that interlocking. At f/8 and a shooting distance comfortable to most birds, I can resolve a vane and its barbs, but the fine barbule detail is blurred away. The shaft and each barb may be sharp relative to each other, however there is a considerable amount of detail that can't be resolved at all at this resolution, or if it is, its rather blurry. At a bird-comfortable shooting distance, barbule detail can be over 100 lp/mm. At f/4, my resolving power increases to around 170 lp/mm, allowing me to resolve much more of that fine barbule detail than I could at f/8 (only limited by sensor resolution...which is where high res. APS-C sensors like Canon's 116 lp/mm (luminance) 18mp sensor or Sony's 129 lp/mm (luminance) 24mp sensor). Now, not only are the spine and barbs sharp...they are sharper than they would have been at f/8 due to the improved acutance and microcontrast offered by a wider diffraction-limited aperture, and
a finer level of detail is now resolving, adding even more to the impression of overall image sharpness.
The same rough effect would exhibit for any kind of photography that aims to expose fine detail. Insect macro photography definitely benefits, as insects contain even more fine detail than birds. When it comes to macro photography, due to extension
, your effective aperture is often much smaller than indicated by your F-Number (in the case of the Canon MP-E 65mm Macro, at full extension the effective aperture can be f/96 or greater if you really stop down to maximize DOF.) Its for this very reason that focus stacking has become a popular mechanism for maximizing DOF and sharpness in macro photos...you can NEVER resolve as much fine detail as sharply at an effective f/96 as you can at say an effective f/22...diffraction simply won't allow it.) The same general rules would apply to portraiture as well...not quite as much fine detail, but still plenty to think about. Sharpness in hair, eye lashes, even iris detail (if your doing head shots) can all benefit from improved resolution at wider apertures...assuming other artistic goals don't override sharpness (which entirely depends on your style.) Many heat shot photographers resort to intriguing post-processing techniques to improve eye detail (many of which produce great results)...but there is no real substitute to finely resolved real, natural iris detail that you can achieve at f/5.6 on a high res (say D800) sensor IMHO.
One aperture can be sharper than another when it increases acutance and microcontrast, and when it resolves finer detail than another clearly...for the depth of field that is in focus (which, admittedly, may be very thin...but thats beside the point when talking purely about resolution...about sharpness.)