As far as auto focus response is concerned, and like issues, it has to an evaluation in consideration of which camera body and its auto-focus system and the auto-focus support that is a part of a particular lens. Any generalizations on these issue can be argued against on a case by case basis for a particular brand camera and whether it may be a Sigma, Tamron, etc., lens.

I have been really happy with sigma stuff,and I am seeing more and more pro types using them, although if you are shooting nikon or canon I dont think you can go wrong with thier high end glass.. but its like anything else, know what you need and dont buy a 200/f8 lens when you are trying to shoot sports at night while sitting next to bob euker.. spend the extra to get a lens thats fast enough or long enough (or both) and you will be much happier than buying the so called best lens out there and its wrong for your style and everything looks like crap because you hate your lens. a friend of mine bought a hassy but couldnt afford anymore lenses and tried shooting everything with the 80mm and then cropping (he had enough resolution to play with in his mind) to zoom or passing on really cool wide angle stuff. he just got in a rut and his stuff looked like it was a point and shoot iso 800 disposable camera. he hated lugging it around and while he's a good photographer and had one of the absolute best cameras made, it didnt work for his needs, so the look suffered more than if he had a mediocre camera and lens. again, sigma gets my 1st vote and others out there are good too. stay away from camera store off brands like quantaray. they are just factory seconds/shortcuts that are rebadged from other makers at a lower price but really are bad for high end use. most consumers dont know the difference though.

Funny I hear third party in articles and in countries, what then is a Second Party whaatever? lol

But, I find if one sticks to the name, non-camera maker's lenses they are usually up to par. And, so notice, that lenses of the higher quality are more expensive using the non-camera makers lenses. Thing is with the non-camera makers many times offer a larger variety of lens styles and focal lengths. Sometimes it is good to get hands on experience suggestions for lenses, even the camera makers. good hunting.

You will notice even when lenses are compared in many articles, the final decision is left to the buyer -- they will not go out on a limb and say buy thus and so. So, after reading the learned articles it is always the same ... use your judgement.

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Funny I hear third party in articles and in countries, what then is a Second Party whaatever? lo

The manufacturer is the 1st party, you are the 2nd party, the other vendor is the 3rd party.