Focal length is the distance between the lens of a camera or optical device and the image sensor when the subject is in focus. It is usually measured in millimeters (mm) and determines how much of a scene will be captured. A shorter focal length, like 18mm, provides a wider view, while a longer focal length, such as 200mm, allows for a closer view of distant subjects. Focal length affects the perspective and depth of field in an image. Lenses with a longer focal length compress the background and foreground, making them appear closer together, while shorter focal lengths create a more expansive view. Understanding focal length helps photographers choose the right lens for their desired composition and effect.