>... Previously, it was giving me sharp corners for all sorts of photos. Then, after a long, bumpy car ride, the problem appeared ...
So, it was presumably damaged.
>Sent it in for repair. Came back 'within spec'. The problem was still there.
I think that says more about the company than the optical formula.
>So I tested it against test charts on a wall ...
In the past I also used use wall charts, but I found a better & easier way. I'm fortunately that I have a mountain ridge at (effectively) "infinity" and clear air. So I do diagonal shots of the ridge and look at the sharpness across the field, in an "X" pattern. That is when I gave up on Canon and switched to prime Leica M glass on a Sony body (with the KoloriVision thin cover-glass conversion). The optical issues disappeared. It's hard being a perfectionist, but I found a solution that works for me.
> ... one or two other corners so out-of-focus that even stopping down at f/11 or shooting at any subject distance doesn't fix it.
I sold a camera from a major manufacturer that had that problem. I have no idea if it was a terrible optical formula or bad assembly. But it was useless for me.
(BTW, the founder of the small company, which became part of Perken Elmer, and build the Hubble told me that the original flaw was an element that had been installed incorrectly -- flipped over. I have no idea if that was true. This guy had a house on Emerald Bay, overlooking a popular beach just north of Laguna Beach in California. In his living room was a 10" reflector telescope [not much longer than it was wide] -- aimed down at the beach. Hmm ...)
Paul
www.PaulRoark.com [/quote]