What this calculator does
Thin lens equation works out image distance and magnification for a lens of known focal length. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
Formula1/f = 1/dₒ + 1/dᵢ; m = −dᵢ/dₒ
The inputs explained
| Field | What to enter |
|---|---|
| Focal length (cm) | A number, measured in cm. Starts at 10. |
| Object distance (cm) | A number, measured in cm. Starts at 25. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with focal length
Every other input is held at the calculator’s starting values while focal length varies. Select any row to load that scenario into the calculator.
How the answer changes with focal length