Sorry, OT a bit more. If Nikon and Tamron both offering the same
kind of lenses, let's say for example 17-35mm f/2.8, Nikkor or
Tamron will be better? 
Usually, except for exceptional cases it is the original SLR
manufacturers themselve that offer the best lens, because they know
their own cameras bodies and can design from that point onward. And
it's the original brand lens that is offering the best quality(over
price, of course) because they have their own proprietery
technologies such as Canon's UltraSonic Motor.
The benifits of a 3rd party lens is the cheaper cost compred to
the original's.Without the proprietary technologies of course, the
bare specs is still cheaper anyway. For instance, a tamron 17-50mm
F/2.8 cost only $500 compared to Canon's 17-55 2.8 which is more
than twice that price.
That's about it. Just the bare specs and not picture quality..
Which is why it's pretty popular with beginners due to their low
cost but not with professionals because of their lower quality
compared to Canon and Nikon's best lens.
