The Marco RT-300 Refractor has cylinder power up to + or - 6.00D, sphere power from +16.75 to -19.00. The accessory knob is positioned next to the cylinder power/axis knob and all controls are within easy working proximity. The unit has fully synchronized cross-cylinders. Exact PD measurement is easy with PD alignment lenses, and with a red lens in one bank and a green lens in the other, the need for special red/green glasses is eliminated.