In which type of astigmatism is the horizontal focal length shorter, and the correcting lens axis is within 30° of the 90° Meridian?

The type of astigmatism referred to in the question is against the rule astigmatism. In this condition, the horizontal focal length is shorter than the vertical focal length. This occurs when the principal meridians of the eye are oriented such that the steeper meridian is positioned horizontally, which leads to the characteristic visual distortions associated with this type of astigmatism.

In terms of lens correction, the axis of the correcting lens needs to be within 30° of the 90° Meridian. This orientation allows the cylindrical component of the lens to counteract the effects of the astigmatism, effectively providing clearer vision. Understanding this relationship is critical for opticians as it informs them on how to prescribe corrective lenses that align with the patient's specific visual needs.

Various other types of astigmatism include with-the-rule astigmatism, where the steeper meridian lies vertically, regular astigmatism characterized by consistent meridian placements, and irregular astigmatism that results from an uneven curvature of the cornea. Each type has distinct characteristics that necessitate different approaches for correction, but it is against the rule astigmatism that accurately matches the criteria set forth in the question.