One: Loupes made with a single lens may add flashes of color to image.  They are useful when color identification is not required. Three: A triplet loupe is a magnifier that contains three lenses placed together in such a way that distortion and color problems are corrected. Ask your Rov what number is needed for what use. Depending on the application, the magnification needs may be different. These loupes are recommended by many for Bedikas Tolaim, Bedkias Arba Minim, and other activities that require magnification and clarity of an image.