In calculating the prescription for a rigid lens, what is the first step?

The first step in calculating the prescription for a rigid lens involves transposing the patient's prescription into minus cylinder form. This is fundamental because many rigid gas permeable (RGP) lenses are designed using the minus cylinder form. This format allows practitioners to more easily address the principal meridians of the patient's refractive error and ensure accurate fitting, especially for astigmatic prescriptions.

Transposing to minus cylinder form involves changing the prescription format that may initially be in plus cylinder form, where the cylinder power is positive. This conversion is not only essential for clarity but also necessary for proper lens design, as it affects how the corrected vision aligns with the fitting process of the contact lens. Starting with minus cylinder allows for a more systematic approach in fitting the lens and ensuring optimal visual outcomes.

The other options, while relevant to the broader process of lens design and fitting, are not the initial step in prescription calculation for rigid lenses. Calculating the diameter of the lens, determining the lens material, or converting to plus cylinder form come into play later in the lens fitting process once the initial prescription has been properly formatted to minus cylinder.