My 50/1.4 focuses perfectly spot on at near field (say about 3m). I can use f/1.4 and get very sharp images at center frame at this distance. However, in far field (say >10m), the lens tends to back focus. Will calibration help? I thought the way calibration would work was that it would shift the focus at all distances. If the far field was fixed to focus perfectly, then the near field would end up front focusing. Is this correct?