I am sure all who have studied ophthalmic and physiological optics will have encountered a diagram similar to figure 1. This shows a pseudo-isometric view of an astigmatic pencil of light. I am also sure, in view of this journal’s target readership that it requires little explanation. It depicts the ray paths along the principal meridians in astigmatic refraction corresponding to a distant point object source (not shown) located on the central optical axis.
Clinical regular astigmatism of the eye is characterised by where the retinal plane intercepts the astigmatic ray pencil. The traditional classification of astigmatism acknowledges five types of astigmatism:
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