The lens equation describes cameras, eyes, magnifying glasses and projectors.
How to use the Thin Lens Equation Calculator
- Focal length
- Object distance
Results update as you type, and every field accepts the units shown in its menu.
How it works
Solve for the image distance from the focal length and object distance, then divide to get magnification.
Inputs explained
| Field | What to enter |
|---|---|
| Focal length | Units: mm, cm, m |
| Object distance | Units: m, cm, mm |
What each result means
- Image distance = 1÷(1÷[Focal length] – 1÷[Object distance])
- Magnification = -[Image distance]÷[Object distance]
- Lens power = 1÷[Focal length]
Example values
How the main result changes when Focal length changes and everything else stays at the example values above:
| Focal length (mm) | Image distance (mm) |
|---|---|
| 25 | 25.316 |
| 37.5 | 38.217 |
| 50 | 51.282 |
| 62.5 | 64.516 |
| 75 | 77.922 |
| 100 | 105.263 |
Formula
1/f = 1/dₒ + 1/dᵢm = −dᵢ ÷ dₒWorked example
Inputs — Focal length: 50 mm; Object distance: 2 m.
Results — Image distance = 51.282 mm; Magnification = -0.02564; Lens power = 20 diopters.
Frequently asked questions
What does a negative image distance mean?
The image is virtual, on the same side as the object.
What is a diopter?
Lens power, the reciprocal of focal length in metres.
What formula does the Thin Lens Equation Calculator use?
1/f = 1/dₒ + 1/dᵢ.