I'm currently using a Nikon D7000 with two lenses: a Nikon 18-200 vr lens (version 1), and a Nikon 35mm 1.8 prime. I'm really interested in getting a new lens for a couple of reasons.
I love the versatility of my 18-200, and I've managed the weight alright. I like having one lens that can do everything. Yes it has a good deal of distortion in its range, but it can be corrected pretty easily, I think. My first kit was a D40 with a 18-55 and 55-200, and it drove me crazy having to switch between the two lenses. So when I upgraded to the D7000 I got a good deal on the 18-200 from a friend (only $200).
Camera and lens together, I saw vast improvements over my D40 kit. Where the 18-200 suffers most is sharpness and speed. It's not very good in low light since its a f/3.5. But the sharpness (or lack of it) is what annoys me. I just feel like it should be sharper than it is. I notice it primarily in my landscapes. It's possible that I'm expecting too much out of it.
I love my 35mm f/1.8. It was affordable, sharp, and fast. The only complaint that I have with it is that it just makes me feel a little crowded. It feels too tight for landscapes, architecture, and indoor photography, which are what I mainly shoot. So I usually just end up using it for a portrait lens since it has good bokeh.
On my recent trip to London, Scotland, and Paris I shot approximately 2,000 photos. So I filtered the the images in Bridge by focal length, and found I spend about 60% of my time at 18mm and about 20% of my time at 35mm with my f/1.8 lens. So that leads me to think that I'd enjoy using a wide angle zoom lens.
I'd also like something 2.8 or lower because I enjoy astrophotography as well. So from what I've found my best options are:
1. Tokina AT-X 11-20 f/2.8 (under $500 new)
2. Sigma 18-35mm ART f/1.8 (around $500 used)
3. Nikon 12-24mm f/4 (around $500 new)
I'd obviously prefer Nikon, but they don't have an affordable wide angle f/2.8 zoom. Any opinions? Like I said, I shoot primarily landscape, architecture, indoor, and some astrophotography. And at some point I plan to get Nikons 50mm f/1.8 to add to my collection.
What do you think?
Edit: if it helps your opinions at all, while I was in Scotland I took a slip with my 18-200. It hit the rim of the filter on a boulder, jamming the filter in place, and making the internals work with a bit of a grind. So I plan to have it repaired this summer for approximately $200. The glass is undamaged, but maybe when Nikon Repair takes it apart they'll correct its sharpness issue?
The Tokina is the only one on your list that'll make you happy. The Sigma isn't wide enough and the Nikon isn't fast enough. I've shot the Northern Lights, in Alaska, with an older version of the Tokina (11-16) and it did a great job.
I have d7100 and am on the lookout for a new lens as well. I have done extensive research and concluded the Tokina is the best choice. I plan to go full frame in the future so aim to get the Pro FX, and if money allows on your end, I recommend that. The sensor will be right in the sweet spot of the lens and it becomes roughly a 15-26 mm lens, after crop factor. Good luck!
I shoot with the Tokina AT-X 11-20 f/2.8 and it's an awesome lens. It's definitively a step below any of my Canon L series lenses, but at the price point, it gives great bang for your buck. You can make some great images with it.
My biggest piece of advice, should you purchase it, is to compose your images such that you can crop the edges of your photo. Like many lenses, it's just not as sharp at the edges. For portraiture, the distortion is unacceptable at the edges, but again, proper composition can alleviate those issues.
I too shoot the Tokina and find it is sharp at every stop... even wide open at f2.8. this image is at 11mm f2.8
I would do the Tokina AT-X 11-20 f/2.8 or the Nikon 12-24mm f/4. I like the ultrawide in the event I want to shoot a sweeping shot. At 11 or 12mm....you get some nice images.