You can't paint with such a broad brush.

You can not assume that all Tamron or Sigma lenses are inferior to Nikon lenses.

You will find that people who examine and review lenses, sometimes will prefer other brands of lenses over Nikon lenses.

It usually depends upon the type of lens involved.

Here is one site that reviews lenses:

http://www.photozone.de/8Reviews/index.html

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