How this calculator works
Moving a lens changes the vergence reaching the eye. The correction is applied independently to the sphere meridian and the sphere-plus-cylinder meridian, then the cylinder is reconstructed from their difference. Positive movement here is towards the eye. The cylinder axis is unchanged by this ideal translation. Results are unrounded optical powers and do not account for available products, corneal fit, tear lenses or toric lens rotation.
Formula
Fnew = Fold/[1 - (dinitial - dfinal)Fold]. New cylinder = converted(sphere + cylinder) - converted(sphere).
Worked example
Moving a -10 D spherical lens from 10 mm to the corneal plane gives -9.09091 D. A +10 D lens with the same movement becomes +11.11111 D.
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Frequently asked questions
Can I order lenses from this result?
The output is an ideal optical conversion. A clinician must establish fit, material, available power and the final prescription.
Should I round each meridian first?
Keep full precision during the calculation. Rounding individual meridians early changes the resulting cylinder.
Why do plus and minus powers change differently?
The signed power appears in the denominator, so moving closer strengthens the required plus power and reduces the magnitude of minus power.