The ZI Diopter, -3 Correction Lens from Zeiss Ikon is for modern Zeiss rangefinder cameras and provides near or far-sighted users a clear viewfinder image without the use of eyeglasses. The diopter correction lens fits into the eyepiece holder of the camera.
To determine which diopter is best for you, consult with your eye physician. Or, you can try a pair of over-the-counter correction lenses typically found in drugstores. Find a pair that permits you to see sharply at approximately 6.5' (2 meters), which resembles the range of a viewfinder. This will provide you a rough estimate as to which diopter is correct for you.