Focal Length Calculator

Work out focal length instantly with clear inputs, formula shown and shareable results.

About Focal Length Calculator

The thin lens equation 1/f = 1/do + 1/di ties focal length to the object and image positions. An object 0.3 m out imaged at 0.15 m implies a 0.1 m focal length, or 10 dioptres. The magnification is -di/do, so this arrangement gives a real, inverted image half the object's size.

Formulas

Thin lens equation

1/f = 1/do + 1/di

Magnification

m = -di / do

Worked example

With the default values below, Focal Length Calculator returns Focal Length of 0.10000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Object distanceminimum 0 m: 0.3 m
Image distanceminimum 0 m: 0.15 m

Results

Focal Length: 0.10000 m
Lens Power: 10.000 D
Magnification: -0.5000
Same Focal Length In Centimetres: 10.000

Step by step

1Values used
2Thin lens equation
3Magnification
4Focal Length= 0.10000 m
5Lens Power= 10.000 D
6Magnification= -0.5000
7Same Focal Length In Centimetres= 10.000

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What does a negative magnification mean?

The image is inverted. Real images from converging lenses always are, which is why a camera sensor sees an upside-down image.

Where does the image sit for a distant object?

At the focal point. As do tends to infinity the 1/do term vanishes and di becomes f.