Had an 8-15 for about 9 months and noticed that the vignetting is not even when zoomed out wide. It has more on the right side than the left and the lower left looks the least. I have a 60D and 5DIII and it happens to both. Thinking of dropping off the lens at Canon's local repair center but want to make sure this isn't normal. Sample image below from 60D. It looks worse on the FF. What do you think? Needs service? Thanks!