"Which lens should I buy?"

This has got to be one of the most FAQ's on the Canon SLR Talk forum at DPReview, and little wonder. Lens selection is arcane, the variety is huge, and there's a lot of money involved. Especially someone on their first SLR, or otherwise thinking seriously about lens choice for the first time, it is overwhelming.

The first commmon mistake SLR newbies make is trying to duplicate the capabilities of their superzoom-equipped digicam on the SLR. This is a bad idea: with very few (and expensive!) exceptions, superzooms are optically mediocre to downright bad and involve other trade-offs as well. A cheap superzoom on a DSLR is usually a big waste of money, as it throws out most of the image quality potential of the DSLR that you already paid for.

The second mistake is to be seduced by an individual, exceptionally capable lens... in Canon's case, usually an "L" lens, or if you're a prime-o-phile, something like the "sharpest lens in Canon's stable," the 100/2.8 Macro. This leads easily to overspending: you pay through the nose for capabilities you really may not need, such as weather sealing, an extra stop or two of brightness, or the convenience of not having to use extension rings on a cheaper lens... and into the bargain you usually get a big, heavy chunk of metal and glass you may not be that thrilled about carrying around.

The trick is to think in terms of line-ups, not individual lenses. Three or four lenses can already give you a very versatile stable that beats a superzoom to a bloody pulp both in terms of optical quality and usability. It also has the advantage of being tailorable to your personal requirements: if you enjoy landscapes and architecture, invest more in the wide end; if you shoot birds, put the dollars in the tele end -- and if you do street and situational, weight the normal range.