Available for all Guard-Dog Purebred and Bones models, this lightweight insert can be easily adapted to become a cost effective prescription solution. *Simply take the insert to your eye doctor and have them fill it with your prescription. Product Limitations: CR-39 Lens Material ANSI Z87.1-2010 Certified Spherical Range ±4.00D Cylinder Range ±2.00D Prism Distance To 3° PD or Pupil Distance must be submitted with every order Multifocal Add Power ±4.00D. No Progressive Lenses. No Full-Width Bifocals. No Double Segments.