The focal length of a lens is the distance between the lens's optical center (or principal point) and the image sensor or film when the lens is focused at infinity. In simple terms, it is the distance from the lens to the point where parallel rays of light converge or appear to diverge after passing through the lens.
For converging lenses (convex lenses), which are thicker in the center, the focal length is considered positive. For diverging lenses (concave lenses), which are thinner in the center, the focal length is considered negative.
The focal length of a lens is a crucial parameter that determines the lens's magnifying power and its field of view. Lenses with shorter focal lengths generally have wider fields of view and provide more magnification, making them suitable for capturing a broader scene or distant objects. On the other hand, lenses with longer focal lengths offer narrower fields of view and are often used for bringing distant subjects closer, such as in telephoto photography.
In photography and optics, the focal length is commonly measured in millimeters (mm). Understanding the focal length is essential for photographers and videographers to choose the right lens for a particular shot and achieve the desired composition and perspective.