How to convert diopters to focal length?

A lens of focal length f has the “optical power” \phi = \frac{1}{f}

As the focal length is given in mm, the optical power is measured in units of \frac{1}{mm}. Converting the optical power to units of \frac{1}{m} results in “diopters”.

A lens of 4 diopters has a optical power of \frac{4}{m} = \frac{4}{1 m} = \frac{4}{1000mm}.
So it has a focal length of \frac{1000mm}{4} = 250mm.
Correspondingly a lens of f=100mm has an optical power of \phi = \frac{1}{f} = \frac{1}{100mm} = \frac{10}{1m} = 10 diopters

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