If you're lucky enough to own an SLR camera, then I think you should also consider getting a fixed-length 50mm lens... and here's why.
First, a little background. 50mm is a special focal length in photography; a 50mm lens is also referred to as a "normal" lens. The reason for this is that, on a 35mm film camera, the perspective you get with a 50mm lens is the same as human vision. You can look through the viewfinder with one eye, and keep the other eye open, and you should be able to see normally.
So why did I mention a 35mm film camera? Well, 50mm isn't actually "normal" on our camera (a Canon 20D) or probably yours. The reason for this is that the sensor in most digital SLRs is not as large as 35mm film. Instead, our digital cameras use a sensor format called APS-C which is less than half the size of 35mm film.
The smaller sensor size has a magnification effect that's referred to as a "crop factor". APS-C has a crop factor of 1.6, meaning that a 50mm lens on our camera has the same field of view as a longer 80mm lens on film. To truly get a "normal" perspective on our camera, we'd need a lens with a focal length of about 31mm. Nonetheless, the 50mm lens has remained a very popular length for photographers.
|Aside: Why are the sensors smaller in digital cameras?|
Well, professional-level digital SLRs, such as the Canon 5D, do actually have "full frame" (35mm) sensors... and they cost more than twice as much as the next camera down. The reason for this is pretty simple--the cost of manufacturing semiconductor chips is pretty directly related to their size. You can only fit so many chips on a wafer, and larger chips means fewer chips per wafer. Creating cameras that use a smaller-sized sensor makes them affordable enough that you and I can have one.
One of the biggest advantages to a full-frame sensor is that it can have the same number of megapixels as a smaller sensor, but without having to pack them in as tightly. This makes a big difference in the noise performance of the sensor. A shot from a Canon 5D at ISO 1600 will look a lot cleaner than it does on our 20D at the same ISO. Another difference, which relates to the crop factor, is that full-frame sensors have a shallower depth-of-field for a given aperture, making it easier to achieve artistic blurred backgrounds.
Ok, so now we know why 50mm is a (historically?) significant focal length, but why would you want a fixed-length lens? Don't you want the flexibility to zoom in and out? Isn't being stuck at one focal length a huge pain?
There are actually a number of advantages to using a fixed-length lens (also called a "prime" lens) over a zoom.
A fixed-length lens is much simpler to design and construct than a zoom, and this has two important consequences. The first is that they can have much better performance than zooms. The simpler design and less lens elements means that they produce some of the sharpest images, and can have larger apertures (for low light and shallow depth-of-field) than any zoom. The second is that they can be made cheaper--much cheaper. To put it in perspective, my wife enjoys photographing people (especially our son), and would love to own the Canon 24-70mm f/2.8 lens.
Canon 24-70mm f/2.8
That lens alone costs about $1,300. Guess how much our 50mm f/1.8 lens cost us? $90! And note the difference there in maximum aperture--the $90 50mm can let it in more light (1 1/3 stops) than the $1,300 24-70mm zoom! I'd also point out that our 50mm only weighs 4.6oz, while the 24-70mm weighs 2.7lbs. That's half-a-pound more than our camera body, and literally almost ten times as heavy as the 50mm lens. That zoom lens is a beast!
Canon 50mm f/1.8
The fixed-length of the lens may seem limiting, but it can actually have some benefits to your work. It forces you to work within a single perspective, and to move yourself around and see things from different angles in order to frame your shot. The limitation can actually help inspire creativity!
All-in-all, the 50mm f/1.8 lens is one of the greatest bargains in all of photography. You get an incredibly powerful creative tool at a very low price (that is, relative to the crazy expensive world of photography).
I should point out that Canon actually makes 3 different 50mm lens. Amazon currently sells the f/1.8 model for $98, the f/1.4 for $375, and the f/1.2 for a cool $1,500. One thing that you'll notice about the f/1.8 (and probably an important factor in its cost) is that the housing is made of plastic. It's noticeable and not very sexy, but the savings are worth it.
If you decide to buy a 50mm lens, you can use any of the Amazon links in this post to buy it and I'll get a little commission for it. Or you can go straight to Amazon just to spite me :). Either way, go buy one and let it inspire you!!!