The thin lens equation relates the distance between a lens and and object to the distance to the image that it forms.

$$ \frac{1}{f}=\frac{1}{u}+\frac{1}{v} $$

Here, $u$ is the object plane distance, $v$ is the image plane distance, and $f$ is the focal length of the lens. If these terms sound confusing, make sure to read over Ray Diagrams.