Focal length意思

"Focal length" is a term used in optics, particularly in the context of lenses, to describe the distance from the optical center of a lens to the sensor or film plane at which the lens can focus to form a sharp image of an object at infinity. In other words, it is the distance between the lens and the image it produces.

For a simple lens, the focal length is the distance at which the lens can focus a beam of collimated light to a point. For a compound lens system, such as a camera lens, the focal length is the distance from the sensor to the point where light rays converge to a minimum size.

The focal length is usually measured in millimeters (mm) and is an important characteristic of a lens, as it affects the field of view (angle of view), the magnification, and the depth of field of the lens. A shorter focal length (wide-angle lens) provides a broader field of view, while a longer focal length (telephoto lens) provides a narrower field of view but can zoom in on distant objects.

In photography and filmmaking, focal length is also used to describe the properties of lenses, and it is an important factor in choosing the right lens for a particular shot or application.