Thin Lens Focal Length Calculator
Use the thin-lens equation to relate object distance, image distance and focal length in a simple optical setup.
How to use this calculator
- Enter positive object and real-image distances in millimetres.
- Check that every input uses the stated units and covers the same period, population or project scope.
- Change one assumption at a time when comparing scenarios.
Formula used by lens focal length calculator
1/f = 1/u + 1/v; f = uv/(u+v)
- 1/f = 1/u + 1/v; f = uv/(u+v)
- It assumes a thin lens and positive distances to a real image; virtual images and signed-distance conventions need another setup.
- Inputs are validated against the tool-specific constraints; displayed results are rounded for readability.
Worked examples
Worked example
For an object distance of 300 mm and real-image distance of 150 mm, the focal length is 100 mm.
The calculator returns focal length, optical power in dioptres and absolute lateral magnification.
The calculator returns focal length, optical power in dioptres and absolute lateral magnification.
Understanding the result
Use the thin-lens equation to relate object distance, image distance and focal length in a simple optical setup.
It assumes a thin lens and positive distances to a real image; virtual images and signed-distance conventions need another setup.
Important notes
It assumes a thin lens and positive distances to a real image; virtual images and signed-distance conventions need another setup.
Treat the result as an estimate for the assumptions entered; verify important specifications against the relevant source or professional guidance.
Frequently asked questions
What is a dioptre?
Optical power in dioptres is the reciprocal focal length in metres.
Does this cover thick lenses?
No. Thick-lens principal planes and lens-system effects are not represented.
How should I use this result?
Enter positive object and real-image distances in millimetres.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.