Brand lenses (Nikkor/canon) vs off brand (Sigma/tamron/ECT)?

I'm sure it comes down to a specific lens but what do you Gain or lose when switching to off-brand lenses? Are they better quality, cheaper, better focal lengths?

Nikkor lenses have higher resale value.

First is that the Sigma lenses are not af-s compatible, meaning if you decide to get a newer body in the future they won't auto focus.

28-70 at the wide end is closer to 42 mm which is not really a wide lens.

The 70-300 of the Sigma does not have VR.

http://comparefroggy.tumblr.com/post/57157688462/canon-ef-70-200mm-f-2-8-usm-l-is-lens

I always trust the brands. I'm a Canon guy and I own one Tamron lens, the 18-200.It has aberration and focus issues so I've not purchased another Tamron… I'm sticking with Canon.

While I normally buy Nikon lenses, some of the 3rd party lenses are pretty good, and even exceed the camera brand lenses. This is especially true with the wider angle DX lenses.

A few notable Sigma DX lenses are:

Sigma 8-16mm f/4.5~5.6
Sigma 17-35 f/1.8
Sigma 17-50 f/2.8

Unfortunately, Nikon (or Canon for that matter) do not make many high end DX lenses. The Nikon 17-55mm f/2.8 is probably the closest when it comes to a premium lens for DX. But the Sigma 17-50mm f/2.8 is actually rated higher at less cost.

Also, Tokina has a wonderful 11-16mm f/2.8, which beats anything Nikon and Canon have.

If you want premium lenses for a DX or cropped camera, you just about have to go 3rd party.

When you get into other areas, especially full-frame, then the 3rd party lenses are not as good. Still, for a sports lens, the Sigma 70-200mm f/2.8 is half the price of either Nikon or Canon equivalents. The lenses are not quite as good optically, but they are a good value.

And the comment that Sigma makes no AF-S equivalent lenses is false. Most of the modern Sigma lenses do have internal focus motors. The only mfg you have to watch out for is Tokina. In Tokina lingo, DX lenses are AF (requiring a Nikon camera with a focus motor), but their DXII (DX2) lens for Nikon do have internal focus motors.

However, you always have to be careful when buying 3rd party lenses as you may run a risk of future compatibility.

When you buy original (note the term), you get assured with almost perfect (yes, it's not 100%) compatibility. If you insist on going with the cheaper third-party (again, note the term) brand, there will be compatibility issues sooner or later.

Original is generally better but more expensive. Third-party is usually cheaper. Focal lengths are almost similar. The best third-party brand is Tokina which may cost almost like the original. Sigma and Tamron follows.

Optical quality is generally better with original but if you want the best, get a manual focus, non-zooming lens from Zeiss, Schneider or Voigtlander.