“sturm Conoid 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. Sturm’s Conoid/Interval: Etiology of Sturm’s Conoid : Focus of Sturm’s Conoid AC/to The Types of Astigmatism:

1. Sturm’s Conoid
Md . Azizul Islam
Associate Optometrist
Oculoplasty Department ( IIEI&H )

2. 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”
Sturm’s Conoid
In simple words, we can say “sturm Conoid 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.

3. So we know that Sturm conoid is an optical
condition in which 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 focal interval.
N.B:The distance between two focal points is
called sturm’s conoid interval.
Sturm’s Conoid/Interval
 The focal interval of sturm: The distance between sagittal and tangential focal lines
is called the focal interval of sturm.
 Circle of least confusion/diffusion: The smallest cross-section of a blur circle
between the two focal lines formed by the astigmatic lens.

4.  At point A, the vertical rays [ V ] , the vertical
rays are converging more than the horizontal
rays [ H ]; so the section here is horizontal oval
or an oblate ellipse.
 At point B ( first focus), the vertcial rays have
come to a focus while the horizontal rays are
still converging & so they form a horizontal
line. (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. (horizontal oval).
Sturm’s Conoid CNTD..

5.  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 diffusion. (circle of least
diffusion).
 At point E ,the divergence of vertical rays
is more than the convergence of
horizontal rays; so the section here is a
vertical oval. (divergence of vertical
rays>> convergence of horizontal rays).
Sturm’s Conoid CNTD..

6.  At point F; ( second focus), the
horizontal rays have come to a focus
while the vertical rays are divergent.
So a vertical line is formed here.
(vertical line at second focus.)
 Beyond f ( as at point G ) : both
horizontal and vertical rays are
diverging and so the section will
always be a vertical oval or prolate
ellipse.
 The distance between the two foci (B
& F) is called the focal interval of
sturm.
Sturm’s Conoid CNTD..

7.  Corneal Causes
 It occurs due to abnormality of curvature of cornea. (Most common
cause of astigmatism.)
 E.g eyelid pressure, pterygium, corneal scars, corneal degeneration,
keratoconus,mild corneal opacities.
 Lenticular causes:
 It is comparatively rare.
It may be-
 Curvatural --- lenticonus
 Positional-----congenital tilting & traumatic subluxation of lens.
 Index----developing cataract/ nuclear sclerosis/ index astigmatism.
Etiology of Sturm’s Conoid

8. Focus of Sturm’s Conoid AC/to The
Types of Astigmatism
 With The Rule Astigmatism :
 Vertical meridian of cornea is more curved.
 Myopic power is more in vertical meridian.
 Eg : -3.0D cyl x 180˚
 Against The Rule Astigmatism :
 Horizontal meridian of cornea is more
curved.
 Myopic power is more in horizontal
meridian.
 Eg : -3.0D cyl x 90˚

9.  Oblique Astigmatism:
 axis is not on or near 90˚ or 180˚
 Range of axis lies in between 120˚-150˚ & 60˚-
30˚
 Eg : -3.0D cyl x 45˚
 Bioblique Astigmatism:
 The meridians are not perpendicular
 Eg : -3.0D cyl x 90˚/ -2.0 D cyl x 110˚
Focus of Sturm’s Conoid AC/to The
Types of Astigmatism

10.  Simple Astigmatism:
 One focal point on retina and the other front or behind
 Of 2 types simple myopic and simple hyperopic
 Compound Astigmatism:
 Both the 2 focal points are in front or behind the retina
 Of 2 types compound myopic and hyperopic
 Mixed Astigmatism:
 One focal point in front and other behind the retina
Focus of Sturm’s Conoid AC/to The
Types of Astigmatism

11. IIEI&H