Good advice from Scott...
Without spending a lot, you aren't going to get a zoom lens that can do it all.
If you have the original 18-55, the lens was revised and improved considerably in the later IS version (better image quality). Now it's been improved a little more with the IS STM version (quieter and more accurate AF). This is still a "kit" lens, somewhat plasticky and inexpensive.... and all versions are slower f3.5-5.6 aperture.
If you are willing to buy used, a decent, but very affordable lens you might consider for general purpose shooting is the EF 28-135 IS USM. This lens has been sold a lot in kit with 30D through 60D, sometimes even 7D. So there are lots of them around and you can pick up a lightly used copy pretty cheap... $250 or less off Craigslist (or similar) is often pretty easy to find. It's not a typical kit lens... better built (though still not "top of the line"), fast and accurate USM focus drive, close focusing so it can be used for some near macro (get macro extension tubes and use those to get even closer). It has IS, to help get steady shots, and image quality is quite good.
One drawback is that the 28-135 is an older design dating back to the days of film, and 28mm isn't wide angle on a crop sensor camera such as your XTi. A more modern model is the EF-S 18-135 IS (doesn't have USM, but the latest version has STM focus drive). Both these lenses sell for upwards of $450 US new, when bought separately from a camera. But bought in kit they often only add $200 or so to the price of the camera alone. So people offer them up pretty cheaply used.
Now, the above zooms are f3.5 to f5.6... same as the lens you presently have. But this makes them more affordable and if you complement the "walk-around" zoom with a fast prime lens or two, you might find it able to do all you need.
For portraits and low light, you might get one of the lenses Scott suggests. The 50/1.4 certainly is a good choice. The 86/1.8 is another great portrait lens. For wider view, the Canon 28/1.8 is good, too... though we often don't think of this sort of focal length for portraiture, in particular (yes, Scott is right, shooting portraits you have to be careful about getting too close with a wider lens). There is also the Sigma 30/1.4.
And, yes, for a dual purpose lens you might consider a short macro lens such as the Canon EF-S 60/2.8. The Tamron 60/2.0 macro might be even better for portraits, since it's a full stop faster. Often with portraiture you want a lens with a large aperture, to be able to blur down the background a lot, to visually separate the subject from the background. The Canon has USM focus, which probably will be faster than the Tamron in most situations. Both these lenses are on sale right now, but still are well over $400 US.
An inexpensive option is the Canon EF 50/1.8... It's little more than $100 used and is a popular entry level lens. The EF 50/1.4 is a better lens, yes, with USM, nicer background blur, considerably better build quality, richer contrast and nicer color rendition... but for the money the 50/1.8 is great for occasional portrait shots. For many poeple, it's their first introduction to faster prime lenses (as opposed to zooms, which are f2.8 at their fastest).
Now for landscape photography we often want a wider lens. There are techniques using standard and even telephoto for the puprose... but in general most people think of wide, when they are shooting landscapes. And, of the above, only the 18-135 is moderately wide angle. That's the same as your current lens, though, at it's widest. So you might just keep what you've got, for that purpose. Eventually you might want wider... and there are a number of possibilities, though most will cost $500 or more new. The Canon EF-S 10-22mm is one of the best, but is also one of the more expensive. There are also Tokina 12-24/4 and 11-16/2.8, Tamron 10-24mm, Sigma 8-16mm, and 10-20mm (two versions). There just aren't a lot of prime lenses that are truly wide angle on a crop sensor camera like yours. One fairly affordable prime that's pretty wide is the Rokinon/Samyang 14mm (also sells under Bower, Vivitar and ProOptic brand names)... but it's manual focus and manual aperture control only.
However, again, for occasional landscape shots you might just keep using what you've already got.... Try stopping it down to f8 or f11, to optimize the image quality with it.
For landscapes and possibly some other things, you might want a circular polarizer filter (cuts glare, makes blue skies bluer and enriches other colors as well), but in general don't use filters on your lenses is my recommendation. Also get a matched lens hood and use it, for each of your lenses.
Your camera is a "APS-C crop sensor model". It is fully compatible with all Canon EF and EF-S lenses. So no worries there. Any third party lens made for Canon EOS/EF mount will fit fine, too... some are designated "crop only" while others are "full frame"... but your camera can use both types just fine.
The only thing I would note if you shop for used gear, some older third party lenses have incompatible electronics, with later Canon camera models. For example, I have an older Sigma 28-75mm that works fine on EOS-3 film cameras and 10D digital, but will not focus (locks up and causes an error) on later Canon camera models. So you have to be a bit cautious about older, third party lenses. Some could be fixed by their manufacturer, but many (including the 28-75 I've got), just aren't worth it. Sigma seemed to have the most of these compatibility issues. Tamron also had a few. I haven't heard of any such issues with Tokina lenses. And there are no such compatibility issues with OEM Canon EF and EF-S lenses and your camera.
Have fun learning to use your new camera!
Alan Myers
San Jose, Calif.
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, lenses and accessories.
