1st time I sent in my lens was for possible fringing. It took a month to get back, with them finding no fringing. LOL BUT, they were "kind" enough to tighten my lens for me even though it was in brand new condition. They tightened it to the EXTREME! Which through off my focus.
So, back it went. A month and a half later, they fixed it and sent it back. It seems ok now, and this was the result of my testing. I would say it is FINALLY fixed!
=)
I hardly EVER shoot at 2.8, but was thrilled with the result. Thank goodness my boy stands like a statue for me now! LOL