A lens of focal length f has the “optical power” =
As the focal length is given in mm, the optical power is measured in units of . Converting the optical power to units of
results in “diopters”.
A lens of 4 diopters has a optical power of
.
So it has a focal length of
.

So it has a focal length of

Correspondingly a lens of f=100mm has an optical power of 
