Question
I'll be travelling to Paris and I'll be bringing me with a Canon T2i/550D, a Canon EF-S 18-200mm f/3,5-5,6 IS and a Tokina 11-16mm f/2,8 (I could also bring a Canon EF 50mm f/1,8 II). I bought the Tokina yesterday so I have no experience with it or using such a wide angle lens and I'll have only a weekend to try it prior to the trip.
I'm an amateur still learning everytime I use my camera so my question might sound basic but in what situation/landmarks should I use these lenses when in Paris? Also, any tips on using my new ultra wide angle lens would be appreciated.
[Edit: Reading the comments, I realize now that the question was really not clear at all. My ititial thought was more about what lens should I bring when going to see monument A or museum B but thanks the experience of this forum amazing contributors, the real question is more about composition and what do I really want to capture and for that, there are no clear answers but many thanks for letting me see the issue from an artistic point of view instead of a technical point of view.]
Answer
Choosing the right lens
It looks like you have a pretty solid range of focal lengths covering everything from 11mm through 200mm. Not a bad set of lenses to work with. One thing that should be noted is that lugging around all that gear might get tiresome after a while, so you might want to think about picking a particular lens to bring if you don't want to go lugging weight around.
As to when to use which lens, I guess there are a few factors that go into that decision: what you want to capture, how much your scene should encompass, and how much reach you need to achieve the previous two. Once you have decided on these three things, and chosen a lens, you should try to achieve the composition you desire. You may find that you end up wanting to photograph a single scene with more than one lens, and thats where art and personal style will come into play. So, without further ado...
Answering the questions
What scene do you wish to capture?
Before you can pick a lens, you need to decide what scene you wish to capture. You have three lenses that cover a fairly broad range of common focal length "buckets": ultra-wide & wide angle, standard/normal angle, and long/telephoto. As a general rule of thumb, the wider you go, the more encompassing your shots can be, and the longer (narrower) you go the less encompassing and more targeted your shots can be. It should also be noted that the maximum aperture of a lens also plays a role here, as wider apertures allow thinner DOF and blurrier backgrounds.
Some of the first things you should think about is what do you want to capture? Are you interested in photographing an entire city skyline at night? Do you want to capture a portrait of friends or family, with a nice creamy backdrop to guide the eye? Do you want to reach out over a noisy foreground and single out something in the distance? Do you want to capture a broad scene with great depth, but focus on a particular subject in the foreground?
How much of the world do you wish to encompass in a single scene?
A single photograph can be a lot of things...can encompass a broad, detailed scene with a lot of interesting elements, or a narrow, targeted, specific scene that focuses on a single subject...or something in-between those two. This is the scope of the photography you wish to take. There are different ways to achieve the scope you desire as well.
In the case of capturing a city skyline, your scope is very broad. Your intention is to bring in a considerable amount of the world you see before you, and keep as much of the detail present as you can. The desire is to create a broadly encompassing shot, which is probably best served by a wide angle lens. Ultra-wide and wide angles, such as 11-24mm, will help you bring in that entire city skyline, along with the waterfront reflecting it in the foreground (i.e. Marina Bay, Singapore @ 22mm). You can put a compositional twist on this scene, and bring in not only a broad scene at a distance, but also include strong foreground element: Photographing the city skyline from the corner of a bridge that stretches out across that waterfront foreground. Again, a wide-angle lens is going to be your friend here. In addition to allowing your photos to encompass a broad scope, wide angle lenses also allow you to simultaneously "get in close", and draw in a critical foreground element (i.e. Bridge in the Fog @ 20mm). Another element of scope is architecture itself. Rather than encompassing an entire city skyline, you may wish to capture an entire building in an artistic manner. You mentioned your trip was to Paris, where there are some of the worlds most renown architectural wonders. Another excellent use of wide angle lenses is capturing a building or city landmark, up close, from head to toe. (i.e. Eiffel Tower @ 18mm).
In contrast to scenes that encompass a broad scope are scenes that encompass a narrower scope. Portraits, for one, are usually intended to focus on a single element of a scene. A narrower field of view, and often a less distinct backdrop, can help narrow the scope of your photograph and focus on just one or a few things...such as a portrait of a girlfriend or wife, or maybe a group shot of some friends. The indistinct, yet present, backdrop of Paris might add an intriguing artistic element, yet not distract from the real intent of the photo...your friends and family. Sometimes you may simply notice something intriguing that you want to focus on, without bringing in a lot of other distracting elements. Focal lengths between 35mm and 135mm are often superb for this purpose (i.e. Mission Completed @ 105mm).
Finally, there may be times when you see something in the distance (or even not so distant) that makes for a superb photograph, but is mired by a lot of foreground "noise" and distraction. This is where your telephoto focal lengths and the added reach they provide come in particularly handy. Longer lenses have the ability to greatly reduce the scope of a scene, and focus on a very specific thing.
how much reach you need to photograph the scene you've chosen?
Reach is the final aspect of choosing a lens. When photographing a wide-angle scene, the reach of the lens is in the breadth of scope that it captures. You may find that too much reach is including elements of foreground or periphery that you don't want in your scene, in which case a less-wide angle is probably necessary. If 16mm is capturing parts of a cityscape that detract from your final vision, perhaps switching from the 11-16mm to the 18-200mm will allow you to narrow your scope a bit, and reduce the all-encompassing reach of wide-angle lenses.
On the opposite end of the spectrum, telephoto lenses offer a different kind of reach...the ability to "reach out" and "reach across". When you find that amazing photo of something off in the distance, such as the Eiffel Tower, but you find that your vision is mired by all the chaos of a foreground market, a telephoto lens that can reach out and beyond that noisy foreground, and zero in on just the Eiffel Tower in the distance, is probably the lens you want. A 200mm lens will do well here, such as your 18-200, or as is often the case...the wildly popular and higher quality 70-200mm lenses that many manufacturers offer.
Composing the shot
As a final step, it should be noted that once you have selected a focal length that meets your initial vision, don't forget to take some time to compose. Simply snapping a shot will certainly allow you to record your trip...but it won't necessarily help you tell a story about your trip, engage you in the art form of photography, or create particularly compelling photographs. It doesn't take long, but when you find the lens you need and frame your subject, think about composition...how the scene is divided, how much depth of field you include (or conversely, how blurry the background is), etc. Taking those few moments will help you craft a photographic story about your trip, and create more compelling photographs that you and your friends might hopefully enjoy more for a longer period of time. Some may even be frame-worthy and end up on your walls. ;)
On another note...you should also not limit yourself to a single focal length for any given scene. You may initially decide that using a 200mm focal length to photograph the Eiffel Tower is best. Your 200mm lens also happens to be a general-purpose lens that supports focal lengths that cover the entire range of buckets, from wide, to normal, to long and telephoto. Try out some alternative focal lengths, and compose a few additional shots. Maybe that "noisy" foreground has some intriguing elements in and of itself, and a slightly blurry Eiffel Tower as a backdrop could create an entirely new and different vision of your trip.
Last, after choosing a lens and composing your shots...don't forget to mind your camera settings. There is nothing worse than getting home and finding that you had your ISO far too high and all your photographs ended up horribly noisy, or that your aperture was not what you intended, and that blurry background turned out to be far more detailed and distracting than you desired.
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