# How do You Determine the Focal Length of a Lens?

To determine the focal length of a lens, you must create a sharp image of an object on a screen - with a lens. Then you measure the distance $$b$$ (screen-lens), as well as the distance $$g$$ (object-lens) and can then determine the focal length $$f$$ with the help of the lens equation:
Or you can focus a far away object ($$g ~\rightarrow~ \infty$$: $$1/g ~=~ 0$$), then the image, because of Eq. 1, lies on the screen exactly at the focal length: $$f ~=~ b$$.