rspmd23 wrote:
I didn't mention she's using a Nikon D200
Well, that's VERY important because it pretty much eliminates the latest and greatest Nikkor 24-70mm "VR" from consideration. That's an "E" lens, which means the aperture is purely electronically controlled. And, if I recall correctly, older Nikon bodies such as D90, D200, D3 cannot handle that. If she had a newer camera AND money were no object, it's probably the best lens of this type with VR.... but at $2400 US it's also currently about the most expensive (new models = little or no discounting), even if one of the most "plasticky"!
Most reviewers consider the Canon II and Nikon VR the "best of the best"optically. The Canon led in this regard initially but is 3 or 4 years old now (2012) and lacks stabilization. It is better built and still is the best corrected to have the least distortion of any. It was the best and about the most expensive in the category when it was introduced... "street" price of it today costs about $500 less than when first intro'd. All the other manufacturers (except Sigma) have since introduced new models and stepped up their game. The Nikon has VR and is slightly closer focusing (both are quite close, "nearer macro" than most zooms).
Both the Canon and the Nikon have beautiful background blur. In this respect the Nikon might be the best of the bunch. But, I really have not looked closely at the third party lenses and how they handle out-of-focus areas. It's another thing I'd carefully compare.
Sigma, Tamron and Tokina all offer alternatives that you should research carefully. My biggest concerns with the Tamron and Tokina would be focus speed... I haven't tried their 24-70s so can't really say. But other Tamron and Tokina I have used typically have been slower focusing than equivalent or similar Canon. Also, Tokina doesn't support "full time manual override" with their "focus clutch" design. AFAIK, all the other lenses do. Finally, with any third party lens there is possibility of future incompatibility... The camera manufacturer changes something in future camera models, which works with their own lenses but the third party lens can't handle it. Sigma has had the most problems in this regard (though, granted, they also have made the most different lenses). Tamron has had a few, too. OTOH, both have been pretty good about fixing issues in current-production models, as well as some higher-value older models. I'm aware of no compatibility issues with Tokina... so far.
That said, I have liked Tokina's build quality best among the three major 3rd party lens manufacturers. Some I've used have reminded me of Canon's premium L-series. And some call Tokina's 24-70/2.8 the sharpest of all... even if not the best for focus speed and convenience.
Sigma lenses are typically very well built, too. But their 24-70mm is now one of the oldest models in the category (current HSM version was intro'd in 2008, I think). It does feature HSM focus drive, which is usually pretty comparable to Canon or Nikon's best performing auto focus. The earlier non-HSM version wasn't as fast focusing.
Of the three third party lenses, the Tamron is the only one with stabilization (they call it "VC"). It also uses USD focus drive, which should be similar to Canon or Nikon's best performing, but confirm this for yourself because I can't say from personal experience.
Using any of these lenses on a DX/crop camera such as D200 reduces or largely avoids one of the more common weaknesses of fast, mid-range zoom lenses... softer corners. Those are more likely to be noticeable on full frame, but are cropped away on smaller format cameras. And, "soft corners" may or may be a valid "issue". It's often not much of a concern among more experienced users. Depends upon uses. Soft corners generally don't effect (might even enhance) portraits... but a landscape photographer might feel quite differently.
Personally, for portraiture I like my 24-70mm better on a crop camera, than on full frame. To me, on FF/FX it just seems a little short focal lengths (though ideal for full length shots, couples, etc., such as wedding photography.... which I don't do).
Be careful when reading online reviews and comparisons of 24-70 lenses. There have been multiple versions of most of them.... and no single review I've seen compares all five manufacturers (Nikon, Canon, Sigma, Tamron & Tokina).
Most lenses of this type now use 82mm filters. Older versions used 77mm.
Note: I included the Canon lens here only as point of comparison, even though it's not compatible with a Nikon D200. Same with the latest Nikkor 24-70 VR, which is also incompatible with that camera, though now roughly comparable to the Canon 24-70 II (better, if you consider the Nikkor has VR and the Canon doesn't have IS... worse if you consider ). Canon essentially invented this lens category with their first version in 2002. Nikon's earlier "G" (non-VR) version was close, but not quite as good.
I have not included Sony/Zeiss at all because they're irrelevant... not usable or compatible with any Nikon DSLR.