I'm looking to get a zoom lens for Nikon DSLR. Now, although Nikon are known for the quality of their lenses, I can't justify spending out just short of £500 on a zoom lens that I can get for a third of that price. Now, it may be that quality has to be compromised here but is there really that much difference in the quality of a Tamron or Sigma lens compared t oa Nikon one of the same type?