Quote Originally Posted by Brian Ellis View Post
The effect of diffraction would show up as an overall "softness" of the image, it wouldn't cause only part of the image to be unsharp while other parts are sharp.

Tobasco, your technique sounds fine. I'd check for film "pop" as many others have stated and also a bellows or lens board leak. I had a lens board from which a cable release gizmo had been removed, leaving a small hole in the board. I foolishly didn't think to cover the hole and the resulting photographs looked a lot like what you're posting. So it's probably a long shot but worth checking.
To quote Rodenstock in several of their lens brochures:

"Depending on the reproduction ratio and the depth of the motif (subject), the required depth of field may make further stopping down necessary. In such cases, the sharpness may be reduced due to diffraction - particularly in the center of the image circle."

The above appears in their taking lens brochures.
The following is from their Apo Rodagon-N brochure:

"Sharpness and contrast are at a maximum over the whole image field. While stopping down further will produce a slight increase in sharpness in the extreme corners, the performance in the center is reduced due to the effects of diffraction."