To understand the clinical application of Conoid of Sturm, we need to understand its basic optics.
In simple words, we can say ” Conoid of sturm is just a representation of how rays are refracted through two different powered meridians” (eg: a sphero- cylindrical lens). So, instead of one focal point, they form two focal lines.
- Conoid of sturm is optically defined as,
“A geometrical configuration of light rays emanating from a single point source and refracted by the sphero-cylindrical lens. Also defined as a bundle of rays formed by an astigmatic optical system consisting of a primary focal line (stermi’s) line, a circle of least confusion and a secondary focal line perpendicular to the primary”
When light rays fall on an astigmatic lens (sphero-cylinder lens), light does not focus on to a single point instead forms two or more focal lines. The light rays which hit the lens horizontally form a vertical focal line, so, a point image appears as a vertical line. The light rays which hit vertically form a horizontal focal line, so, here a point image appears as a horizontal line.
- The focal interval of sturm: The distance between sagittal and tangential focal lines is called the focal interval of sturm i.e gap between the vertical and horizontal focal lines. The images formed here are blurred circles of different sizes.
In between these two focal lines, we can find a circle with best overall focus (but still a little blur) called as Circle of least confusion/diffusion. It occurs exactly midpoint between the two focal lines.
- Circle of least confusion/diffusion: The smallest cross-section of a blur circle between the two focal lines formed by the astigmatic lens. Also defined as “At the dioptric midpoint of focal lines the cross-section of sturm’s Conoid appears as a circular patch of light rays with best overall focus.”
Let us see the optical effect of a Conoid of Sturm practically.
Take a high plus cylinder lens(+10.00 or more with some cylinder) and focus the sun on the ground. At one interval you will see a thin line focused, at another, you will see another thin line 90 degrees opposed to the first. Now if the lens is held at a distance halfway between these two foci, you will see a little focused circle i.e circle of least confusion.
- This concept of Conoid of Sturm happens when there is astigmatism present in the eye. It is corrected with a sphero cylinder lens in such a way that the circle of least confusion is placed on the retina making the image clearer.