How do you measure a focal length?

The formula for focal length is given byÊ1 divided by 1/a plus 1/b, where 'a' is the distance to a lamp or light source and 'b' is the distance to the wall. You find the focal length by placing a lens parallel to a white wall and focus a light source through the lens until one gets a clear image. The lamp has to be positioned so that it is close to the optical axis of the lens.