The Conoid of Sturm
The geometric figure that is formed by the rays of light leaving an astigmatic lens is called the conoid of Sturm (Figure 1-29). The circle of least confusion is the circular cross section of the conoid of Sturm that is halfway between the 2 focal lines—in terms of diopters, not linearly. (Notice that the circle of least confusion in Figure 1-30 is closer to the vertical focal line than to the horizontal one.)
Consider again the spherocylinder of Example 1-9. There are 2 focal lines separated by 75 cm. The region between these 2 linear foci is the conoid of Sturm. Starting at the linear focus in the 30° meridian 25 cm from the lens and proceeding farther to the right, the focus changes from a line to an ellipse with the long axis in the 30° meridian. Farther to the right, the ellipse becomes a circle; then another ellipse with the long axis in the 120° meridian, and eventually a line in the 120° meridian.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.