 For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot.For a diverging lens (for example a concave lens), the focal length is negative, and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens.When a lens is used to form an image of some object, the distance from the object to the lens "u", the distance from the lens to the image "v", and the focal length "f" are related by

The focal length of a thin "convex" lens can be easily measured by using it to form an image of a distant light source on a screen.