8. Astigmatism Optics in RGPs & Soft CLs - Contact Lenses I Summer 2026

Vertex Refresher to the Corneal Plane

Spec Plane: -6.00-1.00x180

Vertical: -6.50

Horizontal: -5.62

**There is no longer 1D of astig in the corneal plane

Vertex Refresher to the Corneal Plane

Spec Plane: +6.00-1.00x180

Vertical: +5.25

Horizontal: +6.50

**There is more than 1D of astig at the corneal plane

Vertex Refresher to the Corneal Plane

Spec Plane: +1.00-1.50x180

Vertical: -0.50

Horizontal: +1.00

**There is no change in astig at the corneal plane

Vertex Refresher to the Corneal Plane

Spec Plane: -3.00-1.50x180

Vertical: -4.25

Horizontal: -3.00

**There is only 1.25D of astig at the corneal plane

Vertex Refresher to the Corneal Plane

Spec Plane: +5.00-2.00x090

Vertical: +5.25

Horizontal: +3.00

**There is more astig at the corneal plane

Myopes have _______ astigmatism at the corneal plane

less

Hyperopes have _______ astigmatism at the corneal plane

more

What are the 3 sources of astigmatism in the eye?

-cornea

-crystalline lens

-retinal distortions

What are the 2 most common sources of astigmatism in the eye?

-cornea

-crystalline lens

True or False:

The axis of the crystalline lens can be at a completely axis than the corneal plane

true

With the Rule (WTR) Fluorescein Pattern (Pic)

With the Rule (WTR) Fluorescein Pattern (Pic)

________ Fluorescein Pattern will lid attach very easily

With the Rule (WTR)

What is the fluorescein pattern of a With the Rule (WTR) spherical lens on a toric cornea?

-Pooling in the steeper (vertical) meridian

-Touch in the flatter (horizontal) meridian

Against the Rule (ATR) Fluorescein Pattern (Pic)

Against the Rule (ATR) Fluorescein Pattern (Pic)

Will an Against the Rule (ATR) fluorescein pattern also lid attach very easily?

No -- these patients are not good candidates for spherical RGPs as the lens will likely slide to the L or R off the cornea

What is the fluorescein pattern of a Against the Rule (ATR) spherical lens on a toric cornea?

-Pooling in the steeper (horizontal) meridian

-Touch in the flatter (vertical) meridian

How does a patient get this rx and where does the -3.00 come from? -3.00-1.00x180

Assume the -3.00 is from excessive axial length

How does a patient get this rx and where does the -1.00x180 come from? -3.00-1.00x180

-cornea only

-cornea + crystalline lens

-crystalline lens only

Corneal Only Astigmatism Example (Pic)

Corneal Only Astigmatism Example (Pic)

When astigmatism is CORNEAL ONLY, will all astig be able to be corrected with an RGP lens?

Yes

Corneal & Crystalline Lens Astigmatism Example (Pic)

Corneal & Crystalline Lens Astigmatism Example (Pic)

Crystalline Lens Only Astigmatism Example (Pic)

Crystalline Lens Only Astigmatism Example (Pic)

When the only astig present is from the crystalline lens, will the lacrimal lens have a power?

No -- astig is not from the cornea at all

Will a spherical RGP be a good idea for a patient with only lens cyl?

No

True or False:

All astigmatism in the spec Rx will come from the cornea cylinder

false

Why is knowing that not all astigmatism in the spec Rx will come from the cornea cyl important clinically?

Helpful to compare the two prior to contact lens fitting

Can the corneal cyl and spectacle cyl be at a completely different axes?

Yes

Why may the corneal cyl and spectacle cyl be at a completely different axes?

The crystalline lens cyl is NOT at the same axis as the cornea

Can astigmatism be induced by a contact lens?

Yes

What are the ways in which astigmatism can be induced by a contact lens?

-RGP or scleral lens warpage

-Lens flexure

-Lens decentration

-Oblique astigmatism aberration

-Toric CL or lacrimal lens surfaces

What is lens flexure?

bending of an RGP/scleral lens on the eye (will induce cyl power)

What is Oblique astigmatism aberration?

the contact lens tilts on the cornea to induce oblique rays (minor source)

Having the wrong axis for CLs or lacrimal lens

Obliquely crossed cylinders

When sphero-cylinders DO NOT have their principle meridians at the same orientation, we (can/cannot) simply add or subtract the powers on the power crosses

cannot

Do we usually use M, J0, & J45 clinically to calculate how the cyl/axis that results from a contact lens that is on the eye and is not matching the spec cyl?

No

What do we usually use in clinic when a contact lens that is on the eye and is not matching the spec cyl?

over-refraction

Why do we use over-refraction when a contact lens on the eye isn’t matching the spec cyl?

It saves time to know what to expect on your over-refraction

What is the assumption used for an emmetropic cornea when calculating eye & CLs cyl combinations?

Cornea needs to be +43.00D for a total eye refractive error of 0 (plano)

An astigmatic eye has a _______ refractive error at a specific axis

plus-cylinder

How do we neutralize a plus-cylinder astigmatic eye?

we neutralize it with a minus cylinder at the same axis

What to expect on over-refraction when 2 cylinders are combined with the same axis?

-If the plus cyl power (eye) and the contact/lacrimal lens cyl power are EQUAL in magnitude the residual astigmatism will be 0

What to expect on over-refraction when the contact/lacrimal lens cyl is greater in magnitude than the plus cyl power of the eye?

overcorrection (90º away)

What to expect on over-refraction when the correcting contact/lacrimal lens cyl is lesser in magnitude than excess plus cyl power of the eye? (Note: An astigmatic eye has a plus-cyl ref error at a specific axis and we neutralize it with a minus cylinder at the same axis so we are just talking about power of the eye not the Spec Rx which has diff rules.)

Under-correction (axis will stay at the same position - under-correction always = axis stays at same position)

Plus Cyl Power and Contact/Lacrimal Lens Power Equal in Magnitude (Pic)

Plus Cyl Power and Contact/Lacrimal Lens Power Equal in Magnitude (Pic

If the plus cyl power and contact/lacrimal lens power are equal in magnitude, what is the residual astigmatism?

0

If the contact/lacrimal lens power is greater in magnitude than the corneal cyl power, what is the residual astigmatism?

overcorrection with axis 90º away

If the contact/lacrimal lens power is lesser in magnitude than the corneal cyl power, what is the residual astigmatism?

undercorrection with axis in the same position

Where will the axis be on over-refraction when the contact/lacrimal lens power and corneal power are at DIFFERENT AXES?

45º from the midpoint between the plus cyl axis (axis needed) and correcting cyl axis

OverRefraction w/ Different Axes Example

Plus cyl axis 90º

Contact lens/lacrimal lens cyl axis: 70º

-Midpoint = 80º

-45º towards plus cyl (90º) = 125º

-The over-refraction cyl will be around 125º

When determining power on over-refraction, if 2 cyls are crossed by 30º, the resultant cyl power will be equal to what?

the correcting cyl power (100%)

When determining power on over-refraction, if 2 cyls are crossed by 15º, what will the resultant cyl power be equal to?

50% of the correcting cyl power

When determining power on over-refraction, if 2 cyls are crossed by 5º, what will the resultant cyl power be equal to?

17% of the correcting cylinder power

If a contact lens is rotated by 30º, you should probably just ________

try a different lens

If the lacrimal lens is _____º off from the eye's cylinder axis, a spherical RGP is not the best choice

30

For most lenses in clinic, the cyl power will likely be _______ on over-refraction

very small

Small differences in axis (5º) will mean that the over-refraction axis is ______ from either axis

very far (45º)

Crossed Cylinder Example

Contact Lens Rx Needed: -2.00-1.25x180

Soft Contact (Lacrimal Lens) Power: -2.00-1.25x010

-Crossed Cyls: 010

-Midpoint: 005º

-Expected Over Refraction Axis: x140

-Power on over-refraction: -0.25 or -0.50