Focal length

Focal length is the distance from a lens’s principal plane to its focal point, expressing how strongly it converges or diverges light. In imaging, it helps determine image scale and angle of view.

Thin lens equation: An equation relating an ideal thin lens’s focal length, object distance, and image distance. It connects focal length directly to where a lens forms an image.
Angle of view: The angular width or height of a scene captured by an imaging system. It narrows as focal length increases when the image sensor stays the same size.
F-number: The ratio of a lens’s focal length to the diameter of its entrance pupil. Unlike focal length, it describes aperture relative to focal length and helps characterize exposure.
Optical axis: A line of symmetry through the centers of an optical system’s components. Focal points and focal length are defined along this reference line.
Principal point: A point where the principal plane of an optical system intersects its optical axis. It supplies the reference point from which focal length is measured in a thick lens.
Lens (optics): An optical element that refracts light to converge or diverge rays and form images. Focal length describes a defining imaging property of lenses.
Depth of field: The range of object distances that appear acceptably sharp in an image. Focal length affects depth of field, but the two quantities describe different properties.
Refraction: The change in direction of a wave as it passes between materials with different propagation speeds. Refraction is how lenses redirect light to produce their focal behavior.
Focal point: The point where parallel rays converge after passing through a converging optical system, or appear to diverge from a diverging one. Focal length measures the distance from the lens’s principal plane to this point.
Camera lens: An optical assembly that focuses light onto a camera’s image sensor or film. Its focal length is a central specification for framing and image scale.

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