The first generation of the Tamron 90mm 1:2,5 Macro lenses had a blue spot problem when used on a DSLR. Reason was the completely flat rear element that didn't work well together with the flat sensor. The problem became only visible when the aperture was closed beyond 8.
That is a very good point.
With an afocal setup the lens should be focused at infinity (real infinity - not what the dial says*) and the aperture wide open.
*Place camera on a tripod - focus carefully on the farthest object you can see clearly on the horizon and then tape the lens so that it can't move.