Quote Originally Posted by noblebeast
I used to use the smallest aperture but then read that diffraction can occur at the smallest lens opening on the enlarger as well as the camera..
I wouldn't hesitate to use the smallest f/stop because of the "risk of diffraction."
Any lens designer worth half the powder to blow them to hell will know enough to limit the smallest lens opening available so that diffraction errors will not be a problem. Diffraction is independent of the glass itself: it is a result of the physical size of the lens opening and the distance of aperture to film plane.
That is why short-focus lenses have smallest openings of f/16 and the long, large format lenses go to f/64 or ?

Of course, it is possible to find a lens that is a total disaster.

The more I think of it, the more inclined I am to agree with Les... It does sound like an alignment problem. Is there a uniform loss of sharpness around the periphery of the frame? If the image is sharp along a particular radial axis when the center of the image is in focus, the baseboard is not perpendicular to the optical axis of the lens.