What Is a Focal Length on Cameras?

Camera lenses come in various focal lengths, including wide angle, normal and telephoto.

The term "focal length" does not refer to a camera attribute, but to an attribute of a lens. Anytime you hear someone talking about a wide-angle lens, telephoto lens or referencing a zoom lens, that person is really referring to the focal length of the lens. Lenses come in a multitude of focal lengths, covering everything from the super wide-angle view to the extreme telephoto view.

Focal Length Defined

The focal length of a convex lens -- the standard camera lens -- gets determined by measuring the distance from the center of the lens to the point where parallel light rays converge. In other words, with a convex lens, separate light rays enter near the top and bottom of the first element of the lens. When the light passes through the last element of the lens, the light starts to bend until the separate light rays meet at some point in the rear. For example, a lens with a 400-millimeter focal length measures 400 mm from the center convex lens element to the film or digital sensor plane where the light rays meet.

What It Means

So, what does focal length really mean for the photographer? The focal length of the lens determines two basic things -- the angle of view and the depth of field. The angle of view symbolizes how magnified an image appears in a photograph. On a 35 mm SLR camera or full-frame DSLR camera, a lens with a 50 mm focal length closely represents the angle of view of the human eye. As the focal length increases above 50 mm, the image becomes magnified and the field of view decreases -- i.e., a telephoto lens. As the focal length decreases below 50 mm, the angle of view gets wider and the image appears farther away -- i.e., a wide-angle lens.

Depth of Field

The blurred background in this portrait is a good example of shallow depth of field.

Depth of field represents the other factor affected by the focal length of a lens. Depth of field corresponds to the distance from the farthest to the closest object in a scene that appears reasonably sharp. Typically, a longer focal length lens has a more shallow depth of field than a lens with a shorter focal length. Other factors, such as aperture, camera-to-subject distance and image size, also play a role in determining depth of field. A portrait with a blurry background shows a good example of shallow or limited depth of field.

DSLRs and Focal Length

Modern DSLR cameras come in two sensor formats -- full-frame and compact. Full-frame DSLR cameras have a sensor the same size as a frame of 35 mm film -- 36 mm by 24 mm in size. Compact-sensor DSLR cameras have a sensor less than the size of a frame of 35 mm film. This smaller sensor size creates a crop factor that affects the focal length of the lens in use. For example, because Nikon's smaller sensor DSLR cameras have a 1.5x crop factor, when a 100 mm lens gets placed on one of these cameras, that lens has an effective focal length of 150 mm. In essence, the lens now exhibits the field of view and depth of field characteristics of a 150 mm lens and not a 100 mm lens.

References

• Space Telescope Science Institute: Definition - Focal Length
• Cambridge in Colour: Understanding Camera Lenses
• Mir: Depth of Field
• Nikon USA: Camera Lens Explained