Highest f-stop

[Lance]

I have heard shooting with a lens full open – my 17-40, f4 at f4 for example – might be it’s “weakest point”, and consequently somewhere in the middle f-stops might be it’s sweet spot.



What about the highest f-stop: 17-40 at f22?



What is the effect there, except for extreme dof?

[John Lindsay]

Most lenses have a "sweet spot" 2 stops down from full open, i.e an f2.8 lens will usually be at its sharpest at about f5.6.

Expensive "pro grade" (eg "L") lenses are sharp even wide open, showing a small improvement if any as you stop down. This, together with better colour rendition and better contrast is what you pay big bucks for. Some long telephoto lenses are designed to be sharpest when shot wide open (see http://www.photozone.de for test data on a whole range of lenses). Cheaper "consumer grade" lenses show a marked improvement as you stop down. Most lenses are pretty decent at about f8 to f11.

After f16 for most lenses, diffraction, or the bending of light rays around the now very small diaphragm orifice causes sharpness to fall off again. So almost all lenses will be sharper at f8 than at f22. For close up or macro work, where you want as much DOF as possible, but also good sharpness, f16 is the "sweet spot" in most cases.

[Willem Semmelink]

Some good explanation of the issue here:
http://www.photodo.com/art/Impr13.shtml