why does lens sharpness not increase monotonically with f-stop?
 

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.

Diffraction adds blur at very small aperture openings. It's only noticeable on big enlargements.

Lens Diffraction.

The better DoF calculators will also indicate when you are diffraction limited. For the Canon APS-C bodies it's typically around f/13. So you'll see lenses getting sharper or leveling out up to that point, then just about linearly deceasing above it as diffraction takes hold.

For FF bodies, it sets in at about f/22, iirc.

Of course, lens design is extremely complicated and below the physics-dictated diffraction limit, you may see other behavior. But stopping down from wide open is generally beneficial for 2-3 stops, then it generally flattens out to the diffraction limit. For typical f/2.8 and f/4 lenses, using f/8 is almost always a safe bet. Fast primes can also at their best a stop or two faster.