We have three basic divisions when it comes to classify the different focal lenghts: wide angle lens, normal lens and telephoto lens. They can render the same given subject in a completely different way, altering significantly the message and the mood of the picture, having each type of focal length its own characteristics and pros & cons.
Starting from the shortest focal length (which can go from 11mm in the upcoming full-frame Canon 11-24 f/4 lens up to 35mm) the wide angle lenses are what can I considered "contextual". While they cover a wider field of view (the Nikkor 14-24 f/2.8G cover of field of 114 degrees of view) allowing us to "get more in the frame", composing a photograph even at 24mm is not an easy task as we have to choose a strong primary subject that is interacting with its context, and balanced with possible secondary subjects and its environment. Distortion known as barrel distortion is a common problem as well with ultra wide angles, as the image tends to distort in two situations: when your horizon is not balanced and when you have subjects on the edges of the frame. Aside from the aforementioned issues which can be solved by proper composition and by perspective correction in post-processing, they are excellent options to use for architecture or landscape, normally requiring less aperture to get a closer hyperfocal distance.
The normal focal distance, which is comprised between 45mm and 55mm is the standard, is the closest to what our eyes can see regarding compression and perspective. 50mm has been the weapon of choice of legends of photojournalism like Cartier-Bresson, Robert Capa and other beasts of the mid 20th century, being widely used for street photography as well. I personally consider that is the lens that has to be used when somebody is starting to dig into the world of photography: a fixed lens that makes you walk in and out of the subject in order to compose, never too close but never too far, wide enough to get a rich context around a given subject, cheap and with an aperture wide enough to get good low light performance and good bokeh (a Nikkor 50mm f/1.4D is $330 USD aprox. and a Nikkor 50mm f/1.8D is only $100 USD aprox., both of them brand new). When I started to work with full frame cameras, the only lens that I had available was a Nikkor 50 f/1.8D which I still have and use from time to time, and helped me a lot to struggle and train my creativeness by not having a zoom lens. Even nowadays, most of the shoots that I take with my Nikkor 24-70 f/2.8G are taken in a range of between 45mm and 55mm. Almost having any distortion at all, lenses in this range are something that you should think about seriously if you are thinking to jump into full frame and you don't have enough money to buy a "normal" f/2.8 zoom.
Last but not the least, we have the telephoto lenses. They are quite useful for subject isolation and for its compression. Their hyperfocal distance is far (if you get to have one that can reach infinity) and thin. Significant mention is to be given to the lenses ranging 85mm f/1.8-1.4-1.2 (Canon), as they are the weapon of choice of portrait photography: it's non-existent distortion makes them the perfect option for flattering images and they normally bring a signature bokeh to the image. As I said before, compression while using longer focal distance is in the agenda, being particularly effective in landscape photography. Oposite to the barrel distortion found in wide angle lenses, they can suffer of pincushion distortion, which is also fixable in post-processing.
As I said before, each type of focal length has its personality, its usage, its drawbacks and its unique strengths. And our choice of focal length does not only have to do with the situation we want to photograph, but as strong or even stronger with our own photographic personality.
Good image hunting!
PS: In the next posts, I'll cover with examples what is Compression, Barrel Distortion, Pincushion Distortion and Hyperfocal length.