astig image

Astigmatism and astigmatic imagery

Emmetropia Astigmatism

Astigmatism and astigmatic imagery

Astigmatism

Image is out of focus

In the examples I will discuss today, the astigmatic meridiansare horizontal and vertical like in the picture. The exact horizontalis referred to as “180o” (insteadof 0o) while the vertical is 90o.

In the clinic you will rarely find theastigmatic meridians to be exactly horizontal and vertical.

Blue = represents Vertical meridian

Red = represents Horizontal meridian

Astigmatic Eyes: two principal corneal meridians

• a meridian of greatest corneal power• a meridian of least corneal power

Color Code

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.

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

Imagery in Astigmatic Eyes: Example 1

Looking at the vertical meridian alone!

-5

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

Imagery in Astigmatic Eyes : Example 1

Looking at the horizontal meridian alone!

+3

Imagery in Astigmatic Eyes : Example 1 The vertical and horizontal meridians together

Under normal viewing conditions, the imagery fora 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 imagelook like for this example eye?

-5

+3


The image formed by this uncorrected eye looks like a blur ellipse

BlurEllipse

-5

+3


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

-5

+3


-5

+3

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 thecircle of least confusion (CLC).

For the eye illustrated here, it will


There are practically an infinite number of focallength combinations for the principal meridians so we cant discuss every possible astigmatic eye.

What does the image look like for this eye?

-5

plano


The vertical line focus is on the retina

-5

plano


plano

+3.0


What the image looks like on the retina

plano

+3.0

HorizontalLine Focus


-5.0

-2.0



-5.0

-2.0

A vertically elongated blur ellipse


+1.0

+3.0



+1.0

+3.0

A horizontallyelongated blur ellipse


The “Circle of Least Confusion”

-5.0

+5.0

At the dioptric midpoint between the two foci,the image forms a perfect circle. This is known as thecircle of least confusion (CLC).

In this example eye, the CLC is at the retinal plane


The “Circle of Least Confusion”

-5.0

+5.0

As the name implies, the image is a blur circle

The best visual performance for an uncorrected astigmatism is said to occur when the CLC is on the retina


“Against-the-rule” astigmatism

+3.0

-5.0

The horizontal meridian has relatively greater myopia(more focusing power) than the vertical meridian

This is less common in the general population thanwith-the-rule astigmatism


+3.0

-5.0

What does the image look like?


+3.0

plano



+3.0

plano

HorizontalLine Focus

WTR

ATR

It is very important to note that the appearance of theuncorrected retinal image does NOT provide informationabout the type of astigmatism (WTR, ATR). The retinal imagetells you the relative position of the meridians.

Classifications of Astigmatism

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.

compoundmyopic astig.

simple myopic astig.

mixed astig.

simple hyperopic astig.

compound hyperopic 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)

from Obstfeld, 1982

Retinal Image in uncorrected astigmatic eyes

All the eyes have “against-the-rule”

astigmatism.

C = compoundM = myopic

A= astigmatismS = simple

H = hyperopic

Object is a distant point source.

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