It took me a matter of hours to learn how to shop for a lens, and what all those numbers mean, but it took me years to work up the motivation to actually learn it, on account of my chronic laziness. I bought my digital SLR camera (a Canon XSi) just before Christmas 2008, which was almost three years ago now. At the time, I had recently lost the point-and-shoot camera I had been using for my 365 Daily Nudes project (at the Burning Man festival, along with all the amazing pictures I took at the festival). A few months after losing it, and being forced to fall back on the even crappier point-and-shoot camera I had been using before that one, I rekindled my fading desire to shoot photography. I decided that it was time to upgrade to "prosumer" level (somewhere between mindless consumer and sophisticated professional) and purchase a dSLR camera.
It's one of the best decisions I've ever made (certainly in terms of my photography hobby).
Thanks to the guidance of a tech-savvy friend, I got a good deal on a Canon XSi. The kit lens included in the package was the EF-S 18-55mm f/3.5-5.6 IS zoom lens. As far as I can tell, it's a fantastic multi-purpose beginner's lens. I was upgrading from point-and-shoot quality photography, so the fact that it's not a high-end high-quality lens failed to affect me. Over time, I've begun to desire even better quality optics, but the flip side of that is that the technical quality of my photography has increased by leaps and bounds since my point-and-shoot days - as well as my knowledge and experience, especially after I made the daring move to start shooting in manual (versus auto) mode, and learn how to manipulate exposure.
Meanwhile, my tech-savvy friend, who's always been at least one step ahead of me (my first two digital cameras were old cameras of his that he was trying to get rid of), started recommending the EF 50mm f/1.8 II [prime] lens, which is a good quality lens for its really low price. I spent several months deliberating, and then finally bought it, lured by the promise of better quality pictures, and faster exposures, thanks to the larger aperture (f/1.8 versus the kit lens' f/3.5-5.6). The latter is of particular importance to me, considering that I shoot a lot of medium to low light photography, and I abhor using a flash. (I want a quick exposure to reduce motion blur, but to do that in situations where there's not a lot of light, you need a larger aperture to suck in as much light as possible).
The 50mm lens delivered on its promises. It's fast, and I really like how much better the pictures look compared to those taken with my kit lens. However, the one significant drawback is that with a 50mm focal length, the angle of view is very narrow - close to the upper range of the 18-55mm zoom that my kit lens sports. So it's like using that lens with it stuck in the "zoomed-in" position (except, of course, with faster exposures and better quality pictures). But for my photography needs, the tight angle of view is hard to deal with. I don't shoot a lot of small objects, and I probably shoot most of my photos indoors, where there's not a lot of room to back up and get everything in the frame.
So, I actually use my kit lens more often, switching in the 50mm only when I can get away with it - like if I'm shooting outdoors, where there's more space - although, ironically, there's usually more light, too, which kind of negates the need for faster exposures. And if I think about it, most of my photography is self-portraiture, where I'm using a tripod anyway, so I can afford to deal with slightly longer exposures. With a tripod, hand-shake from holding the camera isn't an issue, and I've become pretty good at holding still for poses. Hence, my kit lens is usually adequate, although there's still the desire to have a better quality lens - one without the drawback of having a tight angle of view like the 50mm has.
However, there's one other area in which I've felt my photography equipment has been considerably lacking, and that's in shooting convention photography. Most convention photography is indoors, where there's not a huge amount of natural light; and since it's not really practical to set up your tripod, you have to shoot hand-held, so quick exposures are important. But in the middle of a crowd, especially when the hallways are narrow and cramped, there's not a lot of room to back up, so you need to shoot fairly close to your subject. Plus, rather than head-and-shoulders portraits (which have never interested me much), you usually want to take full-body portraits to get the whole costume in the shot (and that's not to mention some of the long props congoing cosplayers sometimes wield). So you have to have a lens with a wide angle of view.
As it stands, I have two lenses. One of them has a wide enough angle of view for this type of photography, but it's too slow, and I've been consistently getting pictures that are way too blurry for my standards. The other lens I have is fast enough, but the angle of view is too tight, so I can't get the kind of pictures with it that I want. So now I'm studying the lenses that Canon has available, looking to see if there's something affordable I might be able to get my hands on. What I need is something with a focal length close to the lower range of my zoom lens, which is 18mm, but with an aperture size closer to my 50mm lens, which is f/1.8. Something fast, with a wide angle. But it also has to be cheap...
In an ideal world, I'd get a super expensive lens that is really high quality - for example, something in the luxury series, like the EF 24mm f/1.4L II USM perhaps (and I might as well get a better camera, too, while I'm at it). But those lenses run for upwards of a thousand dollars, and are completely outside of my price range. I'm looking at the EF 28mm f/2.8, which is one of the cheapest Canon lenses on the market (other than the two I already own). It looks to be a reasonable compromise between the lenses I have: faster than my zoom, but wider than my prime. Although I would really like to try it out first before buying, to see if it's wide enough and fast enough, rather than risk being disappointed.
It's tempting to consider either the EF 24mm f/2.8, which is a bit wider than the 28mm f/2.8, or the EF 28mm f/1.8 USM, which is a bit faster, but the former is a hundred dollars more expensive, and the latter is almost double the price of the 28mm f/2.8. I've never been a technical snob, and as long as I can get the pictures I need, with as much or better quality than the equipment I have (which doesn't allow me to get those pictures), I think I'll be satisfied. After all, the pictures you get with the cheap lens you own are going to be better quality than the pictures you don't get with the lens you don't have (because you can't afford it). Right?
Still, though, it'd be nice to try before I buy. Also, I haven't considered the used market (nor third-party lenses), where prices might be more affordable.