Name-Brand VS After-Market Lenses
After carefully making your decision on which DSLR (digital single-lens reflex) or SLR (film single-lens reflex camera) to buy, it's easy to be tempted by the lower costs of second-tier lenses. Brands such as Sigma, Tamron or Tokina offer compatible lenses for a wide range of Nikon and Canon bodies at prices that undercut the name-brand equivalents.
Price is certainly an important and real consideration in making your decision, but I would strongly recommend sticking with name-brand lenses as you outfit your camera.
Typically speaking, you get what you pay for. First, optical quality, which can be good in an after-market lens, is not as consistent. In other words, the production standards are so much higher with name-brand lenses that it's much less likely that you would get a soft/fuzzy lens. I also find that the construction of name-brand lenses is also much stronger.
Another important factor is cost of repair and time for repair. Canon and Nikon repairs, when done by the companies themselves, generally run 3-4 weeks maximum while repairs from second-tier companies can sometimes take as long as six months, and generally run longer than repair times from Canon and Nikon.