The following illustration is not very pretty, but it is difficult to arrange this sort of comparison with a live subject. The illustration flips every three seconds between f/5.6 and f/22. The f/5.6 version should be sharper/more detailed because f/5.6 is somewhere around the sharpest "sweet spot" aperture for that lens, whilst diffraction has a big impact at f/22. However, when I pixel peeped at 1:1, concentrating on the small areas that were in best focus in the f/5.6 version, I wasn't sure they were sharper or more detailed than the same areas in the f/22 version. This may be because the centre of focus shifted slightly between shots and confused the issue. Also, the processing may have had a stronger effect on the less sharp f/22 version. In any case, my eye is not upset by a loss of fine detail in the f/22 version. On the other hand the difference in how much is in focus seems large to me, and I very much prefer the f/22 version with it having so much more in focus, and that is why I use such small apertures for invertebrates.
The situation is different for botanical subjects and for common birds in flight.
What aperture I use for botanical subjects depends on whether I am using stacking or not. If I am using post focus to capture videos for stacking then I use f/2.8, which is maximum aperture on the 60mm macro. This is not the best aperture for sharpness; that is around f/4 to f/5.6 depending on whether you are more concerned with centre or edge sharpness according to the resolution chart here at ephotozine.com. However, I often work hand-held, sometimes getting close to, or hitting, the 1/30 sec lower limit for post focus, and using f/2.8 keeps the shutter speed faster than it would be with a sharper aperture. I find f/2.8 sharp enough for my purposes and as with the loss of sharpness/detail from diffraction, what I am losing would quite likely not be visible at my output sizes anyway.
Of course, the light level is often high enough to be able to get a fast enough shutter speed with a smaller aperture, so I could use the sharper f/4 to f/5.6. However, there is quite a lot to think about for each capture and leaving the aperture at f/2.8 simplifies things a little and I am content to leave it there as one less thing to have to deal with. This is suboptimal in terms of potential image quality, but practical out in the field, and after a certain point with image quality, for me, practicality becomes the most important factor.
And there is another factor too. Using f/2.8 maximises the out of focus rendition of the background. That is good for separating the stacked subject from the unstacked background, but bad in that it is more likely to produce ugly, too fast transitions between in focus and out of focus areas. Also, as desribed below for aperture bracketing, maximising the out of focus rendition of the background may not be optimal (to my eye) for the overall visual appeal of an image. However, these considerations are too difficult (for me) to judge before I see a stack developed on my PC, and I am content enough with what I get from f/2.8 to take what I get from this unoptimised approach of always using f/2.8.
The trouble, for me at least, with optimising each capture in every respect is that it requires a slow, careful, measured, thorough, deliberative approach which can soak up a lot of valuable time out in the field, decreasing the number of subjects/scenes tackled during a session. For me this is a significant consideration in what are often target rich environments.
Also, full spectrum optimisation can increase the opportunity for error. For example, when using post focus for stacking (which involves using JPEGs with "baked in" white balance) I think I get best results by setting the white balance individually for each scene from a grey card. The trouble is that I risk using the wrong white balance, possibly badly wrong, if I move from say a shady scene to a sunny one and forget to reset the white balance (I am working to develop a strong habit of always setting the white balance for each scene as the first step). This approach involves taking the time to do what are redundant white balance settings when I move from one scene to the next with similar illumination, but the white balance is important enough to me to take that hit (which is not serious as I have set up the G9 so setting the white balance from a grey card is quick to do) and to take the risk of sometimes using a badly wrong white balance.
If I am not stacking a botanical scene then the choice of aperture is solely down to the overall visual appeal of the image, especially the balance between the amount of the subject that is in focus (smaller apertures often better) and the way the background in rendered (larger apertures often better). For single images I can optimise the shutter speed and ISO for a particular apertue. However, I generally use aperture bracketing as this is much faster in execution, but this means taking whatever I get in terms of noise. This is because the ISO has to be chosen so as to give the smallest aperture shot a fast enough exposure, and this means that the ISO is almost always higher than would be the case for an individually optimised exposure.
On looking back through some captures that used aperture bracketing it turns out that I rarely use the wide open f/2.8 captures. This is mainly because, usually, not enough of the subject is in focus at f/2.8. Also, I don't always use the capture that (to my eye) just has enough of the subject in focus and which throws the background out of focus more and isolates the subject more than a smaller aperture would. I sometimes prefer versions where the background is less out of focus.
For common birds in flight I use a particular aperture (f/13 on the Canon 70D) and also a particular ISO (ISO 800) as these settings give me a good combination of depth of field and fast shutter speeds on the bright days I do this type of photography.
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