The Leica Correction Lenses M provide optimum eye-to-camera matching, making composing images in the viewfinder more precise and comfortable. Besides the available grades of +/- 0.5, 1, 1.5, 2 and 3 diopters, you can ask your optician to manufacture a lens with individual diopters. The Mounting Frame for Correction Lens is to hold such individual lenses.
Please note that the Leica M-Viewfinder is preset by default to -0.5 diopters. For example, wearing glasses of 1 diopter strength would require a +1.5 diopters correction lens. Provide this information to your optician.