Determining Focal Length
How is focal length determined to be wide, normal, or long? This is an actual measurement that varies with the camera format and can be quite confusing. The longer the focal length, the less the camera sees, as with a telephoto lens; the shorter the focal length, the more the camera sees, as with a wide-angle lens.
On film cameras, the focal length is the measurement of the distance between the center of the lens (technically the optical center) and the film (technically the film plane). On a digital camera, the focal length measures the distance between the lens and the image sensor. In both cases, focal length is measured in millimeters.
To understand focal length, make a hole with your thumb and forefinger. Bring this hole very close to your eye. At this distance the view is like a wide-angle and the distance (focal length) from the hole to your eye is short. Move it out about two inches for a normal lens view and out six inches for a telephoto.
The same parameters determine the wide-angle, normal, and telephoto lenses of digital cameras as of film cameras. However, for quite technical reasons, the actual measured focal length of a digital photography lens is different from 35 mm. To make matters even more confusing, focal length for the same angle of view can vary from digital camera to digital camera depending on the size of the image sensor array. Therefore, to make it simpler for photographers to understand, references to digital cameras' lens focal lengths are generally referred to with the corresponding 35 mm equivalent lens. For instance, the spec sheet for the PowerShot S300 Digital ELPH camera describes the lens as being 5.4 to 16.2 mm with a 35 mm film equivalent of 35 to 105 mm.
