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: \( f ~=~ b \).