What type of astigmatism occurs when the focal lines are perpendicular to each other?

Regular astigmatism is characterized by the focal lines being perpendicular to each other. In this condition, the eye has two principal meridians—the horizontal and vertical—where light is focused. The corneal curvature in regular astigmatism is somewhat symmetrical, allowing the light rays to meet at two distinct points that lie along these meridians, hence creating perpendicular focal lines.

This contrasts with irregular astigmatism, where the principal meridians are not necessarily perpendicular or have varying curvature that affects light refraction in an unpredictable manner. Mixed astigmatism involves both positive and negative cylinder components, which can create complex images, while simple astigmatism relates to situations where one meridian is normal and the other is astigmatic. Regular astigmatism is the most common type and is more straightforward to correct with lenses that compensate for the uniform curvature differences.