What is the formula for predicting the total astigmatism correction at the spectacle plane using modified Javal's Rule (non-modified)?

What is the formula for predicting the total astigmatism correction at the spectacle plane using modified Javal's Rule (non-modified)?

• A. ARx = Ac + (-0.50D x 090)
• B. ARx = 1.25 Ac + (-0.50D x 090)
• C. ARx = Ac + (0.50D x 090)
• D. ARx = Ac + (-0.50D x 180)

Answer: (A)

Javal's rule connects the astigmatism measured at the corneal/eye level to what appears at the spectacle plane by adding a fixed vertex-distance term to the corneal astigmatism. In the non-modified (classic) form, this vertex-distance term is a cylinder of 0.50 diopters oriented at 90 degrees. That yields ARx = Ac + (-0.50 D × 090). The 1.25×Ac version is the modified form, not the non-modified one, and the sign and axis here reflect the standard direction and orientation of the vertex-distance effect. So the total spectacle-plane astigmatism is the corneal astigmatism plus a -0.50 D cylinder at 090.

Javal's rule connects the astigmatism measured at the corneal/eye level to what appears at the spectacle plane by adding a fixed vertex-distance term to the corneal astigmatism. In the non-modified (classic) form, this vertex-distance term is a cylinder of 0.50 diopters oriented at 90 degrees. That yields ARx = Ac + (-0.50 D × 090). The 1.25×Ac version is the modified form, not the non-modified one, and the sign and axis here reflect the standard direction and orientation of the vertex-distance effect. So the total spectacle-plane astigmatism is the corneal astigmatism plus a -0.50 D cylinder at 090.