The DPM-30 is faster to use and more accurate than a PD ruler. It's durability and ease of use will provide you with years of trouble-free service. DPM 30.2Features Include: Cornea reflection light coincidence method Tolerance: .25mm Left/Right distance: 25-40mm Internal green dot focus point Single eye measurement available LCD Display LED illumination Power saving mode: automatic shutoff after 1 min of idle time Measures tops of corneas to edge of lens/frame