Please note, this diopter correction lens will not fit the M10 and M11 generation cameras. The correct diopter can be found here: Leica M10/M11 Correction lens II, -3.0 diopter

Leica Diopter Correction Lenses provide near or far-sighted users a clear viewfinder image without the use of eyeglasses. Ten Diopter Correction Lenses are available, from -3 to +3. 

For the M series. Screws onto viewfinder eyepiece.