Quote Originally Posted by Nicholas Lindan View Post
I should qualify this: Ctein published an article in the early 90's advocating focusing through a blue filter, and claimed that not using a blue filter resulted in a 1/2" to 1" error in focus wrt the paper position. The claim was rebutted in letters to the editor and a subsequent article. I see Ctein no longer makes his original claim.
That limited refutation seems reasonable, of otherwise pretty decent information.

I don't want to open a can of worms but I believe there could be an error of focus with respect to ultraviolet vs visible in my system... which is quite different than the norm (though not an unreasonable system). I use graded paper and an Aristo grid bulb through a Xenar lens... I may have a system that is more susceptible to focus errors due to ultraviolet exposure than the more common tungsten sources used with multigrade paper through an APO lens.

Our Original Poster here, though, seems to have something very unusual going on, a gross error that really sounds like tampering, dropping, or possibly a bowed piece of paper.