What is Vergence?

The vergence of a lens of focal length $f'$ (in meters) is $C = 1/f'$, measured in diopters (symbol D): $1~\text{D} = 1\,\mathrm{m}^{-1}$; diverging lenses have $C < 0$. Opticians prescribe in diopters: vergences of thin lenses in contact simply add (admitted here; derived in the Year 1 volume).