Sturm's conoid is an optical condition where the refractive power of the cornea and lens are not equal in all meridians, resulting in two focal points separated by an interval. This causes the image formed to change shape from oval to line to oval as the object moves from one focal point to the other, passing through a minimum circle of confusion in between where blur is lowest. The diagram shows how the image shape changes at different points along this path.

1. Mayuri Mohite
1st Year B.Optometry

2. STURM’S CONOID
 It is an optical condition in which the refractive power
of cornea and lens is not the same in all meridians.
 Therefore, instead of single focal point there are two
focal points separated by interval of sturm
 This condition is called a sturm’s conoid.

5. Cont..
 At point A – the vertical rays are converging more
than the horizontal rays, so the section here is
horizontal oval
 At point B – (first focus) the vertical rays have come to
a focus while the horizontal rays are still converging
therefore it forms a horizontal line.
 At point C – the vertical rays are diverging and their
divergence is less than the convergence of the
horizontal rays, so a horizontal oval is formed here.

6. Cont..
 At point D – the divergence of vertical rays is exactly
equal to the convergence of the horizontal rays from
the axis.
 So here the section is a circle which is called the
CIRCLE OF LEAST CONFUSION.
 Diagram showing circles of confusion for point source
 too close
 in focus
 too far

7. Cont..
 At point E– the divergence of vertical rays is more
than the convergence of horizontal rays, so the section
here is a vertical oval.
 At point F – (second focus) the horizontal rays have
come to a focus while the vertical rays are divergent.
Therefore a vertical line is formed.
 Beyond F (at point G) – both horizontal and vertical
rays are diverging and so the section will always be a
vertical oval.