Often for landscape photography people want to use a wide angle lens. Not always, there aren't any rules that say you can't use something else. I'm just sayin' it's typically the first thing that comes to mind
One of the best wide angles you can get for use on a T3i is the Canon EF-S 10-22mm USM. It is not an "L" but that doesn't matter.
I suspect what you have been told about using an L-series lens is due to your camera's crop factor, as several other responses have suggested. L-series will work just fine on your camera, but all L-series lenses are "full frame" designs, by definition. As a result, none of the L's will be particularly wide angle on your camera. So when people hear "landscape", they suggest something other than an L-series.
You have four different stated purposes: landscape, sports, low light and portraiture. The whole point of a DSLR such as your T3i is to be able to interchange lenses, to be able to adapt the camera for use in different situations. So I would suggest not to try to do it all with one lens, but to look at several lenses (some of which are Ls), which ultimately should give you better results. For example....
Wide angle landscape: EF-S 10-22mm.
Sports: 70-200/4, 70-200/4L IS, 70-200/2.8, 70-200/2.8L IS II, 100-400L IS, 300/4L IS.
Low light: 24/2.8 IS, 28/1.8, 35/2 IS, 50/1.4, 50/1.8 II, 85/1.8.
Portraiture (candid): 50/1.4, 85/1.8.
Portraiture (studio): EF-S 15-85 IS, EF 28-135 IS, 24-105L, 24-70/4 IS , 24-70/2.8L II.
Portraiture (environmental): 20/2.8, 24/2.8 IS, 28/1.8, 35/2 IS.
General purpose "walk-around" lens: EF-S 15-85 IS... or, depending upon what other lenses you get, EF-S 17-55/2.8 IS, or EF 28-135 IS, 24-105L IS, 24-70/4L IS, 24-70/2.8L II.
There are lots of different ways to put together a lens kit. For example, maybe you just don't need as wide a view as the 10-22mm for your landscape photography. If so, instead you might choose the EF-S 15-85mm as wide enough and because it can double as a walkaround lens and can even be useful for some portraiture.
A "minimal" kit I often carry around with a crop camera is: 10-22mm, 28-135 IS, 300/4 IS w/1.4X teleconverter, and a Tamron 60/2.0 macro/portrait.
When I'm shooting sports, which I mostly do with crop sensor cameras, I usually carry: 10-22mm and 24-70/2.8, but my most used lenses are 70-200/2.8 IS and 300/4 IS. (In certain situations I also use 70-200/4 IS and 300/2.8 IS.)
For portraiture with a crop sensor camera, my favorite lenses are: 28/1.8, 50/1.4, 85/1.8 and 135/2. Especially the 50mm and 85mm.
Your camera can use all EF-S and EF lenses, including all L-series. But just because a lens will fit and work doesn't mean it's the best choice for your purposes. In some cases buying a full frame capable lens for your camera will be wasteful.... FF lenses can be bigger, heavier and more expensive... yet not really give you any better performance, maybe even give less. For example, a 16-35/2.8L II is a wonderful lens on a FF camera, but makes little sense to buy for use on a crop camera. For less money you can either get a lens that's truly wide (EF-S 10-22mm), or a lens that offers slightly wider range of focal lengths and has IS (EF-S 17-55/2.8 IS).
Depth of field actually is not directly effected by different camera sensor sizes. What changes depth of field are: lens focal length, lens aperture, and distance to subject. If you are looking for shallow depth of field (blurring down a background for a portrait, for example), you need a large aperture and longer focal length... and the closer you are to the subject the more shallow depth of field will be. If, on the other hand, you want a lot of depth of field ( for a landscape shot, for example), you want to use a smaller aperture and a shorter focal length.... and the farther you are from the subject (within reason), the deeper depth of field will be.
Now where crop sensor vs full frame does come into effect is that in order to get the same subject framing with a crop sensor we need to stand farther away with any given focal length, or if shooting from the same distance will use a shorter focal length. In either case, the result is "less" depth of field, but it is an indirect result of the different sensor size.
Have fun shopping and learning your new camera!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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