For the last couple of hours I have been reassembling my SK 28mm f/5.6 lens because one of the internal elements worked itself lose on a plane flight. I discovered something that I use to think was way over-exaggerated, the importance of making sure the front half of a large format lens is placed at the exact distance from the rear half. This is usually done through the use of placing paper thin shims at the base of the front half.
In process of taking the lens apart to fix the loose element, I separated the lens from the shutter and dissembled the front half. I noticed three shims in the front with first one falling out. In the process of screwing the front half back into the shutter, that shim got creased and caused the front to be slightly angled. This created a slight loss of focus on one side. I then removed this shim, feeling it would be better without and the focus totally dropped off on the edges. It remained tack sharp in the center of the IC, but out of focus on the edges.
I just got finished flattening the shim and placing it back in position so as to lie flat (not an easy task) and the lens is just as sharp as it was a few days ago.
It turns out that I have been wrong in assuming the shims did not make that much of a difference. In fact they are extremely important.
