Hi,
I didn't say that I always use f/8, I said I strive to use f/8. Why?
- Autofocus is inprecise, so is manual focus (except possibly LV). So I want to have some margin for improper focus
- Stopping down more will loose some resolution/contrast
Sometimes I open up for short DoF and sometimes close down for long DoF. But in general I use long focal lengths if I need short DoF and wide lenses for long DoF.
I have also found that I want my principial subject to be in focus rather than trying to optimize DoF and having everything slightly unsharp.
That still makes no sense to me, as usual with arguments about comparing different formats at equal f-stop (and/or equal ISO speed).
1. For focus precision, what matters in general is being some number of stops below the maximum aperture, the aperture that generates the OOF effects used by manual of automatic focus detection. So lenses of lower minimum f-stops will AF safely at proportionately lower f-stops: a fixed f/8 does not make sense, and the overall trend is for larger format lenses to have higher minimum f-stops, suggesting the need for a higher f-stop to get equally reliable focus.
2. In formats like 35mm or smaller, optimum sharpness as the focal planes these days achieved at below f/8: as I mentioned before, even moderately priced zoom lenses like my Olympus 14-54 and 50-200 seem to be optimal at about f/4 to f/5.6, so that f/8 will in your words "loose some resolution/contrast".
3. On stopping down losing some resolution and contrast: you are apparently saying exactly what I suggested: that sharpness and detail primarily matter to you only at one distance. Is it really the case that most of your photographs have all the interesting subject matter at roughly the same distance from the camera? Or that you can usually control DOF by choice of focal length at a preferred f-stop, which requires either changing your camera position to control FOV, or accepting a FOV dictated by DOF choice. Oh, and accepting perspective relationships dictated by camera position. (Sorry, let's not go there again!)
OK, maybe this is indeed your style, but I doubt that this is relevant to most photographers: I suspect that most of us adjust DOF with f-stop choices, and so for most of us, the two comparisons of greatest practical relevance are:
a) With compositions that are roughly equal in factors including DOF and FOV and perspective, so requiring equal camera position, and focal length and f-stop proportional to format size [as in that PhotoDo comparison.]
b) At optimal f-stop for the particular lens, in your sense of sharpness/resolution/contrast at the focal plane: this has a tendency of meaning at an f-stop that increases somewhat with format size, though in a far less rigid and predictable way than equal DOF comparisons.