How do Sigma lenses for Nikon compare with the actual original Nikon lenses? Are they as good? What about Tamron lenses?

it depends which lenses you are comparing, what you are using the lenses for, what budget you have, whether weight considerations are an issue etc etc

Thanks for your reply... I'm vaguely looking at a 70-200 mm f2.8-5.6 Nikon or Sigma... The Nikon is about 2300 Euros, the Sigma about 1400 Euros! As regards budget, obviously the cheaper the happier I am but I'm too poor to buy cheap (as my grandmother used to say!). I don't want to be disappointed and have to change in a year's time. Less weight is better for me too. Does that help?

As Selkie has pointed out. Your use of the lens will give you the clue. Nikon lenses are the product of thousands of hours of research, top class materials and design, efficient lens coatings to mention just a few. They would be expected to perform to the highest standards. If you are a professional photographer with multi-million pixel optical sensing capability, and of course need the very best in resolution for your job, then the choice is easy. If you need something practical for family use, and which costs a lot less, then choose Sigma or Tamron. They are very good lenses for general use, but don't expect the ultimate in performance. If you are really serious about photography, the cheaper-as-happier will be disappointing. Heavier lenses may indicate glass being used instead of plastic in the construction. Yes the best lenses are expensive, but if you can afford to buy the best, you will be happy and proud to own it. Good luck.

Buy the Tamron or Sigma. Then, every photograph which you come to cherish you'll look at and wonder - "Wow, I'll bet that image would be fantastic if it were made with real Nikon glass". There is no question, if you are going to be making more than just snapshots of the family on easter, then get the real thing.

I totally disagree. I know two amazing professional photographers who use Sigma lenses all the time. One of them in particular does a lot of high level fashion and sports photography, with fast moving subjects and sometimes in bad light conditions. I think the advantage with Nikon is mainly the more solid build but not the optical quality. Of course there is a difference there too, but it is so minimal that it's unlikely to be worth the extra money, which is significant.

Louise - your 'professional' friend must have one magic touch. Every one of his/her pictures would be better with Nikon glass. Have you ever heard about modulation transfer function? Sigma is great - unless you want your photographs to look nice.