This process involves mathematically altering a spectacle prescription written in plus cylinder form to its equivalent in minus cylinder form. This conversion maintains the refractive power of the lens but changes the representation of the astigmatism correction. For instance, a prescription of +2.00 +1.00 x 90 can be reformulated as +3.00 -1.00 x 180.
The practice of rewriting prescriptions offers advantages in lens manufacturing and fitting. It allows for greater compatibility with lens grinding equipment and facilitates the utilization of certain lens designs optimized for a particular cylinder convention. Historically, the industry has shifted between preferred formats, and understanding this interconversion is crucial for practitioners to interpret and work with prescriptions from different sources or eras.
