Which type of astigmatism is defined by consistent relationships between focal points?

Regular astigmatism is characterized by consistent relationships between focal points, meaning that the two principal meridians of the eye are at right angles to each other. This regular arrangement allows for predictable optical characteristics. In individuals with regular astigmatism, the varying curvature of the cornea is stable, resulting in a consistent difference in refraction between the two meridians. This condition can typically be corrected with cylindrical lenses that are oriented appropriately, enabling clearer vision.

In contrast, irregular astigmatism presents an unpredictable arrangement of focal points due to uneven curvature, often resulting from conditions such as keratoconus or trauma. Mixed astigmatism involves one meridian being myopic while the other is hyperopic, leading to a more complex interaction between focal points. Simple astigmatism features one normal and one astigmatic meridian, which does not present the same consistent relationship found in regular astigmatism. Each of these other types of astigmatism possesses distinct characteristics that contrast with the predictability of regular astigmatism.