focale

Focale, in optics and imaging, refers to the focal length of a lens or mirror. In French usage, the term is commonly expressed as distance focale or longueur focale and denotes the distance from the lens’s principal plane to the focal point where light rays converge. The focal length is typically measured in millimeters and is a fundamental specification of a lens.

The focal length has a direct impact on the image: it determines the field of view and the magnification of a scene. Shorter focal lengths yield wider angles and more of the scene, while longer focal lengths provide greater magnification with a narrower view. The choice of focal length interacts with sensor size, as crop factors alter the effective field of view for a given focal length.

Optical relationships are often described by the lens equation for a thin lens: 1/f = 1/v + 1/u, where f is the focal length, u is the object distance, and v is the image distance. For distant objects, 1/f ≈ 1/v, and the sharp image forms near the focal plane, the plane located f units behind the lens. The focal plane is what the sensor or film sits on to capture a sharp image.

Applications: In photography, focal length distinguishes wide-angle, standard, and telephoto lenses, with fixed focal length (prime) lenses offering specific fields of view, and zoom lenses covering a range. In astronomy and telescopy, the focal length of the objective and the eyepiece determines magnification, approximately M ≈ f_objective / f_eyepiece. The f-number (f/D) also relates focal length to aperture and light gathering.

Overall, focal length is a defining optical parameter that links lens design to image size, perspective, and exposure characteristics.