Regular Astigmatism Refractive types

Depending upon the position of the two focal lines in relation to retina, the regular astigmatism is further
classified into three types:

1. Simple astigmatism, wherein the rays are focused on the retina in one meridian and either in front (simple myopic astigmatism – Fig. 1 a) or behind (simple hypermetropic astigmatism  Fig. 1 b "BELOW") the retina in the other meridian.
2. Compound astigmatism. In this type the rays of light in both the meridia are focused either in front or behind the retina and the condition is labelled as compound myopic or compound hypermetropic astigmatism, respectively (Figs. 1 c and d).

3. Mixed astigmatism refers to a condition wherein the light rays in one meridian are focused in front and in other meridian behind the retina (Fig. 1 e). Thus in one meridian eye is myopic and in another hypermetropic. Such patients have comparatively less symptoms as 'circle of least diffusion' is formed on the retina . (FIG 2 BELOW)  

 FIG 1 Types of astigmatism : simple myopic (A); simple
hypermetropic (B); compound myopic (C); compound
hypermetropic (D); and mixed (E).

 

 

 

 

 

Fig.2 Sturm's conoid.