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Astigmatism and astigmatic imagery

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object as a point image on the retinal surface. ... The image on the retina for this eye is a blur ellipse. ... As the name implies, the image is a blur circle ... – PowerPoint PPT presentation

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Title: Astigmatism and astigmatic imagery


1
Astigmatism and astigmatic imagery
Emmetropia Astigmatism
2
Astigmatism and astigmatic imagery
Astigmatism
In the examples I will discuss today, the
astigmatic meridians are horizontal and vertical
like in the picture. The exact horizontal is
referred to as 180o (instead of 0o) while the
vertical is 90o. In the clinic you will rarely
find the astigmatic meridians to be exactly
horizontal and vertical.
Image is out of focus
3
  • Astigmatic Eyes two principal corneal meridians
  • a meridian of greatest corneal power
  • a meridian of least corneal power

Color Code
4
Emmetropic Eyes The vertical and horizontal
principal meridians have the same power and
focus a distant point object as a point image on
the retinal surface.

5
Imagery in Astigmatic Eyes Example 1 Looking
at the vertical meridian alone!
The vertical meridian will produce a horizontal
line focus of a distant point object. In this
example, the vertical meridian focuses a distant
point object in front of the retina. This is a
myopic refractive error. Lets say the vertical
meridian has a refractive error of -5 D
-5
6
Imagery in Astigmatic Eyes Example 1 Looking
at the horizontal meridian alone!
The horizontal meridian will produce a vertical
line focus of a distant point object. In this
example, the horizontal meridian focuses behind
the retina. This is a hyperopic refractive
error. Lets say the horizontal meridian has a
refractive error of 3D
3
7
Imagery in Astigmatic Eyes Example 1
The vertical and horizontal meridians together
Under normal viewing conditions, the imagery
for a point object has contributions from both
principal meridians
The horizontal and vertical line foci are both
far away from the retina. What does the retinal
image look like for this example eye?
8
Imagery in Astigmatic Eyes Example 1 The
image formed by this uncorrected eye looks like a
blur ellipse
Blur Ellipse
9
Imagery in Astigmatic Eyes Example 1
The vertical meridian has greater myopia (more
focusing power) than the horizontal meridian.
This is the most common form of astigmatism
This is known as with-the-rule
astigmatism Next lecture we will talk about
correcting astigmatic eyes
10
Imagery in Astigmatic Eyes Example 1
The image on the retina for this eye is a blur
ellipse. However, at the dioptric midpoint
between the two foci, the image forms a perfect
circle. This is known as the circle of least
confusion (CLC). For the eye illustrated here,
it will
11
Imagery in Astigmatic Eyes Example 2
There are practically an infinite number of
focal length combinations for the principal
meridians so we cant discuss every possible
astigmatic eye. What does the image look like
for this eye?
12
Imagery in Astigmatic Eyes Example 2
The vertical line focus is on the retina
13
Imagery in Astigmatic Eyes Example 3
14
Imagery in Astigmatic Eyes Example 3 What the
image looks like on the retina
15
Imagery in Astigmatic Eyes Example 4
16
Imagery in Astigmatic Eyes Example 4 What the
image looks like on the retina
A vertically elongated blur ellipse
17
Imagery in Astigmatic Eyes Example 5
18
Imagery in Astigmatic Eyes Example 5 What the
image looks like on the retina
A horizontally elongated blur ellipse
19
Imagery in Astigmatic Eyes Example 6 The
Circle of Least Confusion
At the dioptric midpoint between the two
foci, the image forms a perfect circle. This is
known as the circle of least confusion (CLC).
In this example eye, the CLC is at the retinal
plane
20
Imagery in Astigmatic Eyes Example 6 The
Circle of Least Confusion
As the name implies, the image is a blur circle
-5.0
5.0
The best visual performance for an uncorrected
astigmatism is said to occur when the CLC is on
the retina
21
Imagery in Astigmatic Eyes Example
7 Against-the-rule astigmatism
The horizontal meridian has relatively greater
myopia (more focusing power) than the vertical
meridian This is less common in the general
population than with-the-rule astigmatism
22
Imagery in Astigmatic Eyes Example 7
What does the image look like?
23
Imagery in Astigmatic Eyes Example 8
24
Imagery in Astigmatic Eyes Example 8 What the
image looks like on the retina
25
WTR
ATR
It is very important to note that the appearance
of the uncorrected retinal image does NOT provide
information about the type of astigmatism (WTR,
ATR). The retinal image tells you the relative
position of the meridians.
26
Classifications of Astigmatism
compound myopic astig.
Classified on the basis of the positions of the
line foci formed for a distant point source with
respect to the retina and the positions of the
principal meridians.
simple myopic astig.
mixed astig.
Position of the most powerful meridian
with-the-rule near vertical (?30 deg)
against-the-rule near horizontal (?30 deg)
oblique near 45 or 135 (?15 deg)
simple hyperopic astig.
compound hyperopic astig.
from Obstfeld, 1982
27
Retinal Image in uncorrected astigmatic eyes
All the eyes have against-the-rule astigmatism.
C compound M myopic A astigmatism S
simple H hyperopic
Object is a distant point source.
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