I have a canon CN-E 30-300 that can't focus infinity at the infinity mark. I'm able to get crisp infinity focus, but the mark on the lens is slightly before the infinity mark. I'm trying to narrow down where the issue may lie, whether it's a backfocus adjustment or a flange depth issue with my F55. Before I purchased the lens three years ago I had a 300mm Red Prime which had terrible CA so I tried adjusting the flange adjustment on my F55 thinking it might help the issue. Turns out it was just a terrible lens. I made slight adjustments to the adjustment screw and put it back in it's original position when I was done. But I can't help but think the flange is not in it's EXACT factory position. Bottom line, is my infinity issue a backfocus or flange issue?