Quote Originally Posted by Sebastian
Chunk is correct, diffraction will cause the smallest aperture to soften the image.
Thanks Seb. I think I may not be completely correct. After writing that response I felt kinda guilty for spouting off remembered stuff from articles I had read many years ago when first trying to make sense of all this.
A google search on "lens sharpness" included this tutorial on the Luminous Landscape site.
http://www.luminous-landscape.com/tutorials/sharp.shtml
Since I enjoy the humor and mix of technical and practical advice on that site I went there first. In this article he says "Prime lenses usually have larger apertures than zooms, and the laws of optics say that all other things being equal (which they often aren't) a wider aperture lens will be sharper than one with a smaller aperture — the issue being diffraction effects."
Does that mean that a given lens is sharper at f1.4 than f8 or just that an f1.4 lens will be sharper at f11 than an f4 lens will be at f11? I had always thought that a lens was sharpest in the middle of it's aperture range but maybe I'm wrong and it's sharpest wide open? That doesn't seem logical to me with all the aberations hanging around the outer edges where the light is tightly bent.

Anyway I think the article is worth a read. He also points out several of the points that Peter has already made in his reply. There are some links to other related articles on the bottom of the page.

I guess I could read some other more technical sources, but I already have my headache for the day.