Knowing how focal length works helps to decide if you actually need to buy that expensive telephoto or an ultra wide zoom lens.
Zoom lenses provide helpful insight about focal length. Changing the focal length of a lens can be thought of as zooming effect. The greater the focal length the more zoom-in on your subject you get. The shorter the focal length value the wider zoom-out is obtained.
Traditionally focal length is calculated relative to 35mm camera format. Cameras with sensors smaller that 35mm in width have a crop factor that increases the actual focal length for lenses used with these cameras. LensTutorial automatically calculates 35mm equivalent focal length if selected camera sensor is smaller.
“Normal focal” length of a human eye in 35mm equivalent is around 50mm. Everything that is shorter than is considered wide angle lens. Everything that is longer than 50mm is named telephoto lens. Traditionally in photographic literature focal length is abbreviated as f e.g. f=35mm.