What is the primary characteristic of regular astigmatism?

Regular astigmatism is primarily characterized by the presence of two focal lines that lie at right angles to each other. This condition occurs because the cornea or lens has different refractive powers in different meridians, which results in light rays coming into focus at two separate points rather than a single point. This is typically observed in regular astigmatism, as the corneal curvature is symmetrical but varies in steepness from one meridian to another.

The concept of focal lines being oriented at right angles is essential for understanding how regular astigmatism can be corrected with cylindrical lenses, which are designed to counteract the refractive differences between the meridians. This arrangement allows for clear vision in different orientations, as each focal line corresponds to a specific orientation of astigmatism.

Other characteristics mentioned in the options relate to differences in focal points and lens shapes that do not align with the traits of regular astigmatism. For example, the idea of focal points not intersecting relates more to irregular astigmatism, while the notion of focal points intersecting at an angle would usually pertain to different types of astigmatism that do not have the right-angle relationship between meridians found in regular cases.