In the center, at least, Bryan's ISO 12233 chart images show that the lens is very sharp at f/4, at all focal lengths. Ideally, zoom lenses should be tested at their longest focal length, and at 25x that focal length (your 8 foot test fits that criterion). You may have a lens in need of service - if it's new and under warranty, definitely call Canon. If you bought it used, it's still worth a call to Canon Service - they'll give you a free estimate if it needs repair. Honestly, the 24-105mm f/4 is a much better lens than the 15-85mm f/3.5-5.6 - I really think you're better off with the 24-105mm lens, unless you really need the 15-24mm range (in which case, I'd suggest the EF-S 10-22mm).


Personally, I do have a couple of EF-S lenses, but they're lenses for which there's no real L-series equivalent, i.e the 10-22mm f/3.5-4.5 andthe 17-55mm f/2.8 (the latter was a tough debate vs. the 24-70mm f/2.8L, but i wanted the IS and the wider end for my 'walkaround' lens).