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 