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

## Answer #1

Level 2 (suitable for students)

Answered by Alexander Fufaev

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:

**Ideal lens equation**

Formula anchor $$ \begin{align} \frac{1}{f} ~=~ \frac{1}{b} ~+~ \frac{1}{g} \end{align} $$

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: \( f ~=~ b \).