"Against the rule" refers to the corneal astigmatism when?

"Against the rule" astigmatism occurs when the horizontal meridian of the cornea is steeper than the vertical meridian. This means that the curvature of the eye is such that it has a greater curvature (or steeper profile) along the horizontal axis compared to the vertical axis. In practical terms, this would typically lead to light being focused at different points along these two meridians, which can affect vision quality.

In this context, when someone refers to "against the rule" astigmatism, they’re using a convention of describing the orientation of the steepest and flattest portions of the cornea. Since the horizontal meridian is steeper, it contributes to the corrected vision being more favorable in the vertical plane, contrasting with "with the rule" astigmatism, where the vertical meridian is steeper.

Understanding the orientation of astigmatism is important for prescribing corrective lenses, as it informs the axis of the cylindrical power needed to neutralize this refractive error, ultimately leading to improved vision.