What is the equation for residual astigmatism?

What is the equation for residual astigmatism?

• A. Refractive cylinder minus Corneal cylinder
• B. Corneal cylinder minus Refractive cylinder
• C. Refractive sphere minus Corneal sphere
• D. Refractive cylinder plus Corneal cylinder

Answer: (A)

Residual astigmatism is the internal or lenticular astigmatism that remains after accounting for the cornea’s contribution. It is found by subtracting the corneal cylinder from the refractive cylinder, since the refractive cylinder represents the total astigmatic error the eye presents to correction and the corneal cylinder represents the cornea’s share of that error. The remaining difference is the internal astigmatism. For example, if the refractive cylinder is -2.00 x 180 and the corneal cylinder is -1.00 x 180, the residual astigmatism is -1.00 x 180. If axes differ, you’d use vector subtraction to find the true residual, but in this basic form the idea is subtracting corneal from refractive. Other options don’t fit because they either reverse the subtractive relationship, deal with spherical error rather than astigmatism, or add cylinders instead of finding the remaining internal component.

Residual astigmatism is the internal or lenticular astigmatism that remains after accounting for the cornea’s contribution. It is found by subtracting the corneal cylinder from the refractive cylinder, since the refractive cylinder represents the total astigmatic error the eye presents to correction and the corneal cylinder represents the cornea’s share of that error. The remaining difference is the internal astigmatism.

For example, if the refractive cylinder is -2.00 x 180 and the corneal cylinder is -1.00 x 180, the residual astigmatism is -1.00 x 180. If axes differ, you’d use vector subtraction to find the true residual, but in this basic form the idea is subtracting corneal from refractive.

Other options don’t fit because they either reverse the subtractive relationship, deal with spherical error rather than astigmatism, or add cylinders instead of finding the remaining internal component.