Every lens has a focal point, where all the light directed at that lens converges or diverges and the distance between the lens and that point is called the focal length. The radius of the curvature of a lens is exactly twice the focal length. Lenses can create both real and virtual images, real being images that are actually there, virtual images only being able to be seen by looking into the lens. Images are also either inverted or erect, upside down or right side up respectively. The focal length is referred to as F and the radius of curvature 2F. The magnification of any lens can be calculated by subtracting the ratio of the height of the image to the height of the object and the ratio of the distance from the lens of the image to the distance of the object. If the result is a negative number, then it represents the factor of reduction as opposed to the factor of magnification.