The following charts are vertex conversion charts for all sphere and cylinder combinations between spheres values -4.00 to -4.75 and cylinder values between 0.00 and -3.25.
The numbers in the ‘Starting Powers’ (blue) column are typically the numbers you would find on a prescription for glasses.
The numbers in the ‘Back Vertexed Powers’ (green) would serve as a theoretical starting point for converting a glasses prescription to contact lenses.
Please note that these chart are for reference and informational purposes only. They are not meant to determine anything more than the result of the back vertex formula. A prescription for contact lenses requires many more steps, including a physical examination of the contact lenses on your eyes, and can only be issued by a licensed optometrist or optician.
Sphere: -4.00 Cylinder: 0.00 to -3.25
Sphere: -4.25 Cylinder: 0.00 to -3.25
Sphere: -4.50 Cylinder: 0.00 to -3.25
Sphere: -4.75 Cylinder: 0.00 to -3.25