During some some property (real estate) shots today I came across something I didn't know. With the objective of getting images as sharp as possible I had did all the usual prep. Tripod, remote release etc. In this case I was using a quality prime lens so the 'sweet spot' associated with zooms wasn't a factor.
Took two images per scene - f11 & f32, manual focus. The f32 images were, without exception, softer than f11. I had expected them to be sharper providing there was absolutely no movement during exposure. Know f32 is a bit extreme but was useful in establishing a reference point. Both images below are straight out of the camera (Nikon raw).
At first I thought the longer exposure time coupled with a very slight wind may have been a factor. Still possible but I feel unlikely given I took around six scenes with a three bracket exposure on each. Every image showed the same 'softness'.
After a little research I found the issue, which I should have known anyway. I spend a lot of time shooting at small f-stops (2.8 ish) so this doesn't come up. In a nutshell, smaller aperture (larger f-stop number) equals more distortion of light. Very small apertures cause increasing light diffraction. In layman terms, forcing the light through a tiny hole causes light rays to interfere with each other. At this high an f-stop other factors start to play a part - aperture blade profile, camera shake etc.
The zoomed images below shows two different scenes, first image f11 second f32. Images below are 100% JPG exports from Lightroom. Aware this introduces error but am too lazy to export as TIFF then to PNG. PNG support in Lightroom is long overdue!
Takeaway here is there's a cut of point on all lenses where higher f-stop does not equal a sharper image. Some simple tests shooting a series of shots on a tripod aimed at a fine detail subject will show where that is. As a rule of thumb f11 seems to be sensible.