Aspects of Camera Lenses: Focal Length
One of the properties of camera lenses to think about is focal length. In order to take well-focused pictures, there is a length from the lens to the sensor (or, in film cameras, to the flat plane of the film) and that is called the focal length. With a typical 35mm film camera, the image size will fill that frame, but with a digital camera, the image will be about 1-1/2 times smaller than on film. This means that the image will be filled with the center of what would normally be on a 35mm frame; this looks like a cropped image.
Understanding Focal Length
Focal length is the millimeter distance from the focal point on the center of the focal plane to the center of the lens where the subject is in focus. The lens transmits the image from in front of the camera to the sensor or film.
A standard size lens is a 50mm lens, because it approximates the view and focus of the human eye. Without adjusting the focal length, the image will be in focus. Anything shorter than 50mm is considered a wide-angle lens. If it's larger than 50mm, it would be called a zoom lens.