How is residual astigmatism calculated?

• A. Refractive cylinder - corneal cylinder
• B. Refractive cylinder + corneal cylinder
• C. Corneal cylinder - refractive cylinder
• D. Sum of spherical and cylindrical errors

Answer: (A)

Residual astigmatism is the portion of astigmatism that remains after you subtract the cornea’s contribution from the total refractive astigmatism. Conceptually, it shows what internal optics (like the lens or posterior cornea) add to astigmatism.

The best way to quantify it is by taking the refractive cylinder (the astigmatism found in the manifest refraction) and subtracting the corneal cylinder (from keratometry or corneal topography). If both match in axis and power, the residual is zero, meaning all astigmatism is coming from the cornea. If they don’t match, the difference is the internal or residual astigmatism that remains after corneal influence is removed.

For example, if the refractive cylinder is 1.50 D at 90 degrees and the corneal cylinder is 0.75 D at 90 degrees, the residual astigmatism would be 0.75 D at 90 degrees (the internal component). This isn’t found by simply summing powers or by reversing the subtraction; the idea is subtracting corneal from refractive gives what’s left over. If axes differ, you would combine them using a vector approach, but the core idea remains: residual = refractive cylinder minus corneal cylinder.