What type of astigmatism occurs when the vertical focal length is shorter and the axis is within 30° of the 180° Meridian?

With the rule astigmatism refers to a specific type of astigmatism characterized by the orientation of the principal meridians in relation to the horizontal and vertical axes. In this condition, the vertical focal length is shorter than the horizontal focal length, which means that the eye has the strongest refractive power along the vertical axis. The axis being within 30° of the 180° meridian implies that the steepest curvature (or greatest refractive power) is aligned vertically. This alignment leads to blurriness or distortion in vision when viewing horizontal lines, while vertical lines may appear clearer.

The other types of astigmatism mentioned have different characteristics. Against the rule astigmatism would involve a flatter vertical axis, while simple and compound astigmatism describe different relationships in focal lengths, but do not focus specifically on the meridian orientation as it relates to "with the rule." Understanding these distinctions is crucial for proper diagnosis and treatment in optical practices.