# Lens equation – Concave lens

Before we continue, we must learn another equation called the "lens equation".

But it is okay because it is really easy to remember using the following:

Lens equation

IF I DO I DIE

`I/F=I/(do)+I/(di)`

`1/F=1/(do)+1/(di)`

The lens equation is:

`1/F=1/(do)+1/(di)`

Where

`F` | `=` | Principal focal length | |

`do` | `=` | Object distance | |

`di` | `=` | Image distance |

**NOTE:** This works for lenses that are thin but the thicker this lens gets the less accurate the equation becomes. But for general purposes and basic exams, this equation is normally used.