<p>I have been using FDn lenses professionally for about thirty years I bought my DFDn 17mm f4 lens new and it's an excellent ultra wide angle lens but the problem of buying second hand after all these years is you have no way of knowing it's history, and it sounds to me as if some ham fisted amateur repairer has had it apart on his kitchen table and ruined the collimation, because any idiot can dismantle a lens, but to reassemble it correctly and ensure all the elements are parallel to each other and line up to the same central datum line requires the use of an optical bench, and a lazer beam shining through the lens elements to ensure they line up correctly which is a job for for a camera technician.</p>