413. What is the difference between object distance and focal length?
The object distance is simply a measure of the distance between the object and the lens. The image distance is a measure of the distance between the lens and the image that it forms. A positive number for the image distance means that a real image forms. A negative number for the image distance means that a virtual image forms (on the same side of the lens as the object). The image distance depends on both the object distance and the focal length of the lens. The focal length of the lens is a characteristic of the lens itself and doesn't change as the object and image distances change. The focal length is equal to the image distance when the object is very, very distant (e.g. a star). A positive focal length lens (a converging lens) forms a real image of the star at a distance from the lens equal to its focal length. A negative focal length lens (a diverging lens) forms a virtual image of the star at a distance from the lens equal to its focal length.