This image was shot on a APS-C -sensor at a focal length of 35mm. 

1 Normal is Relative
The main job of a lens is to concentrate (focus) light rays so the film or digital sensor can record the scene. Put simply, the distance it needs to make the light rays converge completely is called the focal length.

Lenses that bend light rays very effectively make them converge in a short distance. These short-focal-length lenses take in a very broad spread of light—a wide angle of view. Lenses with long focal lengths (telephotos) bend light much more gently, taking in a narrow field of view.

There’s another factor in play here: The size of the film or sensor. Put a tiny sensor behind a lens and it will take in only a small amount of the image. A big sensor will take in a much bigger area. For this reason, bigger sensors need longer focal-length lenses, and smaller sensors need shorter ones, for the equivalent angle of view.

In between are normal focal lengths, so called because they produce natural-looking perspective at moderate shooting distances. What’s normal? By convention, a focal length a bit longer than the diagonal measurement of the film frame or digital sensor. A full-frame sensor has a diagonal measure of about 43mm, so lenses around 50mm are “normal.” For APS-C-sensor cameras, which have a diagonal of about 28mm, a focal length of 35mm is considered normal. For Four Thirds cameras, 25mm. A typical compact has a normal of about 8mm.

TIP: Set your lens to a normal focal length—even better, mount a normal lens—and force yourself to shoot a variety of subjects. You’ll find you’ll move around a lot more and you will discover different perspectives doing so.