In a Back Toric lens, what should the corneal astigmatism be compared to the refractive astigmatism?

In a Back Toric lens, the relationship between corneal astigmatism and refractive astigmatism is crucial for effective correction. The correct choice indicates that the corneal astigmatism should ideally be about 2/3 the magnitude of the refractive astigmatism for optimal performance of the lens. This ratio ensures that the lens aligns properly with the eye's existing astigmatism, which is essential for providing clear vision and effective correction.

The reason this specific fraction is preferred over others is that it allows the Back Toric lens to effectively compensate for the astigmatic error presented by the cornea while minimizing distortion that could arise from mismatched powers. A corneal astigmatism that is too strong relative to the refractive astigmatism may cause issues like poor lens stability and inadequate visual acuity, while a weaker ratio could lead to undercorrection and persistent astigmatism symptoms.

Thus, understanding this ratio is vital, as it reflects the relationship between the corneal surface and the refractive error, ultimately aiding in the selection of the appropriate toric lens to match the patient's visual needs.