In the old film days it was common to have a camera come with a 50mm lens. This focal length gives you nearly the same field of view as the human eye.
Because a fixed focal-length lens isn't commonly included with a camera these days, a 50mm is often an early purchase choice for many photographers. These lenses are normally inexpensive, small, lightweight, and have a fast aperture (f/1.8 is not unusual, and if you're willing to spend more you can go as fast as f/1.2), making them ideal for shooting in low-light situations.
The standard range goes up to 90mm, which is a popular length for portrait work due to the flattering way it portrays the subject's face without distortion (Figure 3.18) and the unobtrusive working distance it allows between camera and subject.
Figure 3.18. A standard lens is typically in the 50mm to 90mm range. It gives you an angle of view that closely matches the human eye and perspective.