Choosing An Appropriate Focal Length
Learning to choose an appropriate focal length is important to realizing your creative vision.
Lenses come in a variety of focal lengths, wide angle, normal, and telephoto. Wide angle lenses have a wider field of view than normal lenses and telephoto have the narrowest field of view. In other words, wide angle lenses capture the most, side to side, and as the focal length increases the area captured, side to side, decreases. (see figure 1)
Wide Angle Lens
Wide angle lenses will distort the view in such a way that the background is pushed away from the camera. Focal lengths smaller than 35mm are considered wide angle..
Normal lenses will capture the view much like we see it. Focal lengths between about 40 and 58 mm are also considered normal.
Telephoto lenses compress the background making it appear much closer than it is.
Prime vs Zoom Lenses
Prime lenses are those that don't zoom and are a fixed focal length.
Zoom lenses have a variable focal length.
Choosing a focal length
Choosing a focal length takes consideration. You may want to push, or compress, the background, or you may not want to do either. Sometimes you don't have a choice. Maybe the subject is too far away and you are unable to get closer and you have no choice but to use a telephoto lens. Or maybe because of confined space you are forced to use a wide angle lens. But given a choice, you should consider what you wish the image to look like.
See the examples below. I recommend that you go out and practice with different focal lengths to get a feel of how they change the scene.