Which of the following describes the rule astigmatism?

Astigmatism is a common refractive error that occurs due to an irregular shape of the cornea or lens, causing light to refract unevenly. The correct description of rule astigmatism indicates that the steepest meridian lies at 90 degrees. In rule astigmatism, the vertical meridian (90 degrees) is steeper than the horizontal meridian (180 degrees). This characteristic means that patients will typically experience better vision when looking straight ahead, but may struggle with other angles or orientations.

The other options do not accurately describe rule astigmatism. For instance, in rule astigmatism, the flattest meridian does not specifically have to lie at 180 degrees. Furthermore, while it is true that astigmatism can affect vision at all distances, it does not exclusively impact near vision, and curvature is not equal in all directions as it is an inherent characteristic of astigmatism itself. Thus, the specific detail that the steepest meridian lies at 90 degrees is what correctly characterizes rule astigmatism.