dioptre
/daɪˈɑptər/
noun
- A unit of measurement for the refractive power of a lens, equal to the reciprocal of the focal length in meters.
- A camera lens with a power of 4 dioptres can focus on very close objects.
- The doctor prescribed reading glasses with a strength of +2.5 dioptres.
- The optician measured the lens and found it had a power of -3 dioptres for nearsightedness.