Focal length – the fundamental factor

Of the many properties a lens possesses, focal length is the most basic. At any given camera to subject distance, focal length is the sole factor governing image size. To a large extent focal length also influences other lens traits such as maximum aperture, depth of field, angle of view, weight, length, diameter and ease of handling. When a photographer goes shopping for a new lens, it is usually focal length that is of most concern.

Just what is meant by the focal length of a lens? Optical designers have their own precise definition, of course, but for most photographers it is accurate enough to describe focal length as the distance from a lens’ optical center (about where the diaphragm is located) to the camera sensor when focus is set at infinity. From this it is obvious that focal length is strictly an optical measurement. It seldom corresponds with the real physical length of a lens. Furthermore, the actual focal length of a particular lens may not always be identical to the number marked on the barrel. However, any difference is always insignificant, and it in no way affects usefulness of a lens.

Why are focal length and its control of image size so important? It gives us flexibility in choosing a camera position. If we could always conveniently move forward any distance to get a closer view of a faraway object, or move back just as easily when a subject is too big to fit the viewfinder, many of us probably would never buy lenses of different focal lengths. The normal lens or kit lens that comes with a camera would be suitable for all occasions. However in reality an object’s image size is often difficult to control with only one lens. A distant object’s image size is often difficult to control with only one lens. A distant subject may be separated from you by a body of water or some other barrier. Or, the subject may move away if approached too closely. On the other hand, the wall of a room may prevent you from backing up enough to take in a group of people, or a fence or cliff may stop your retreat when you are trying to capture all of a large outdoor scene. The answer to such problems is a change in lens focal length.

A telephoto lens has the focal length that is greater than that of a standard lens. Like a telescope, this type of lens increases image size but cover less of the scene when the focal length is longer. The smaller the angle of view, the more a subject is magnified. A wide angle lens works in a reverse manner. It takes in more of the scene before the camera, but renders everything smaller. The shorter the focal length, the more pronounced the effect. Therefore, without having to change camera position, you can bring distant objects closer with a tele lens. Or, by using a wide angle lens you can reduce the image size of large, nearby objects so that they better fit the picture area.

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