Hey, gang, look at these images: http://forums.dpreview.com/forums/read.asp?forum=1036&message=15110817
Last spring I bought (and returned) an F* 300 f4.5 that had residue on the rear element that looked just like this. The rear of the lens also smelled strongly of mothcake. I suspected that a previous owner had stored the lens near or with mothcake, which outgassed and left a residue.
Joe
