Focal length is the distance between the lens of a camera and the image sensor when the subject is in focus. It is measured in millimeters (mm) and determines how much of a scene is captured. A shorter focal length, like 18mm, provides a wider view, making it ideal for landscapes, while a longer focal length, such as 200mm, allows for close-up shots of distant subjects, like wildlife. Focal lengths also influence the perspective and depth of field in an image. Lenses with longer focal lengths compress the background, making objects appear closer together, while shorter focal lengths create a more expansive view. Understanding focal lengths helps photographers choose the right lens for their desired composition and effect.