The L series lenses are more tightly monitored, therefore its safer to assume you will have a sharp lens out of the box. This is the reason why I only had one go at the tamron 17-50. I initially had gotten this lens and it was soft at allapertures, I had read that many people had to return multiple copies until they were satisfied, so I returned it and purchased the 17-40 instead, and it seems to be as sharp as can be.