Hi all -- just a puzzle I want to understand:

At Dpreview, they have lens sharpness reports where you can cycle through the different f-stops and zoom settings, and see the resulting curves of aberration and sharpness versus position on the image.

What I am trying to apply my elementary understanding of optics to is why sharpness of image can be decrease (be worse) at narrower f-stops (f/22)? I had long assumed that the depth of field governed the sharpness -- i.e. as you stop down the lens and bring more depth of field into focus, all parts of the image would get sharper.

Yet for some lenses, you can see that there is a sharpness maximum (average across the field) for example around f/5.8 and gets worse as you go narrower. (I use the term narrower so not to confuse "smaller" with the f number)

Can anyone explain? Hope I've described this well enough. You can see an example here: http://www.dpreview.com/lensreviews/...ration.xml%3F2

At some level, I understand that an increase in the range of distances that are in focus does not necessarily correlate with the sharpness of the image on the focal plane, but would like a better explanation of it... And as a practical matter, this kind of changes my default behavior of assuming that for sharpest images (aside from desired DOF effects), I would usually stop down as much as possible.