Focal length

Focal length is a term used in optics to describe the distance between the lens and its focus. The focal length of an optical system is a measure of how strongly the system converges or diverges light. In most photography and all telescopes, the focal length is an important measure as it relates to angle of view and image magnification.

Definition

The focal length of a lens is defined as the distance from the lens to the point where light rays coming from a point at infinity converge. This is often represented by the symbol f. For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens.

Types of Focal Length

There are two types of focal lengths: short and long. Short focal lengths have a wider field of view and less magnification, while long focal lengths have a narrower field of view and greater magnification.

Short Focal Length

A lens with a short focal length is known as a wide-angle lens because it captures more of the scene in a single shot. These lenses are often used in landscape and interior photography.

Long Focal Length

A lens with a long focal length is known as a telephoto lens. These lenses magnify the subject, making them appear closer than they actually are. They are often used in wildlife and sports photography.

Focal Length and Aperture

The focal length also affects the aperture, which is the opening in the lens that allows light to enter. The aperture is usually expressed as an f-number (like f/2.8 or f/16), which is the ratio of the focal length to the diameter of the aperture. A smaller f-number means a larger aperture and more light entering the camera.

See Also

• Lens (optics)
• Aperture
• Photography
• Telescope

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  Diagram illustrating the concept of focal length.
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  Diagram of a thick lens showing focal points.
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  Example of effective focal length in photography.
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  Images showing the effect of a thin lens on light.
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  Animation showing the effect of focal length on image distance.
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  Diagram showing the optical power of a lens.