Chapter 6  (Astigmatic lenses)

1. The following are true about the cylindrical lens:

a. it has two meridian of curvature

b. it has no power along its axis

c. it can be used to measure phoria

d. it causes meridian magnification or minification at 90⁰ to its axis

e. it forms a focal line parallel to its axis
 

Answera.F   b.T   c.T   d.T   e.T
A cylindrical lens has one plane surface and the other with curve surface. 
It has no power along its axis. Its power is 90 degrees to the axis and the lens forms a focal line parallel to its axis.
It is used in Maddox rod for the measurement of phoria.
 
 

2. The following are true about the Maddox rod:

a. when the cylindrical lenses within the rod are vertical, a vertical line 
    is seen by the eye when it views a distant white spot through the rod

b. when the cylindrical lenses within the rod are horizontal, a horizontal line 
    is seen by the eye when it views a distant white spot through the rod

c. light incident in the meridian at 90⁰ to axis of Maddox rod is seen as a 
    white line

d. it is used to measure phoria for both near and distance

e. it can be used to measure cyclotorsion
 

Answera.F   b.F   c.F   d.F   e.T

When the cylindrical lenses within the rod are vertical, a horizontal line

is seen by the eye when it views a distant white spot through the rod.
A horizontal line is seen when the lenses are vertical. Light incident in
the meridian at 90 degrees to axis of Maddox rod is focused in front 
of the eye which is too close for the eye to see as a line. Maddox rod
is used to measure distant phoria and Maddox wing for near phoria.
Double Maddox rod can be used to measure cyclotorsion.
 
 

3. Irregular astigmatism:

a. occurs when there are many radii in one meridian

b. can be corrected with spectacle

c. is usually corneal in nature

d. can be caused by different indices of refraction in different portions 
    of the crystalline lens

e. can be treated with laser refractive surgery
 

Answera.T   b.F   c.T   d.T   e.T 
Irregular astigmatism occurs when there are many radii in one meridian.
It is commonly corneal in nature and caused by scar but it can also be 
lenticular in origin caused by different indices of refraction in different 
portions of the crystalline lens. Spectacle can not correct irregular astigmatism. It is treated by substituting a new surface and this can be achieved with a contact lens usually RGP lens, corneal graft or laser surgery.

4. The following prescription has oblique astigmatism:

a.       PL / -2.00X45

b. +1.00 / -0.25 X85

c. +1.25 / +1.00X175

d. -4.00 / -0.25X35

e. -5.00 / -1.00X130
 

Answera.T   b.F   c.F   d.T   e.T
By oblique astigmatism, we mean that the axis of the correcting cylinder
is other than near 90 degrees or 180 degrees (other than with or against the rule astigmatism).

5. The following prescription has against-the-rule astigmatism:

a. +1.00 / +0.25 X 180

b. +1.00 / -025 X 180

c. +1.00 / -0.25 X 90

d. +1.00 / -0.25 X 135

e. -1.00 / +0.25 X 90
 

Answera.T   b.F   c.T   d.F   e.F
In against-the-rule astigmatism, the plus cylinder is at 180 degrees
and the minus cylinder is at 90 degrees.
In with-the-rule astigmatism, the plus cylinder is at 90 degrees and the minus cylinder is at 180 degrees.

6. In against-the-rule astigmatism:

a. the horizontal meridian has more power than the vertical meridian

b. a minus cylinder at 180⁰ will correct the astigmatism

c. a plus cylinder at 180⁰ will correct the astigmatism

d. the astigmatism may be reduced with clear corneal incision at the temporal 
    side during cataract surgery

e. the visual acuity is less affected than with-the-rule astigmatism
 

Answera.T   b.F   c.T   d.T   e.F
In against-the-rule astigmatism, the horizontal meridian has more power than the vertical meridian. A minus cylinder will correct the astigmatism 
if placed at 90 degrees or alternatively a plus cylinder can be used which is placed 180 degrees. Temporal incision will reduce against-the-rule 
astigmatism. The visual acuity is less affected in with-the-rule astigmatism than against-the-rule astigmatism.

7. Simple astigmatism occurs when:

a. one image is focused on the retina and the other is in front of the retina

b. one image is focused on the retina and the other is behind the retina

c. both images are in front of the retina

d. both images are behind the retina

e. one image is in front of the retina and the other is behind the retina

 

Answera.T   b.T   c.F   d.F   e.F
Simple astigmatism occurs when one of the images is on the retina. 
Simple myopic astigmatism occurs when one image is on the retina 
and the other in front of the retina. Simple hypermetropic astigmatism occurs when one image is one retina and the other is behind the retina.
Compound myopic astigmatism occurs when both images are in front
of the retina. Compound hypermetropic astigmatism occurs when both images are behind the retina.
Mixed astigmatism occurs when one image is in front of the retina and the other behind the retina.
 
 

8. The following are true about the conoid of Sturm:

a. it is the three-dimensional image from by a spherocylindrical surface

b. images at the end of the conoid are lines

c. images at the centre of the conoid are circle

d. the circle of least confusion is the centre of the conoid of Sturm

e. the length of the conoid of Sturm increases if the differences between the 
    two foci of a spherocylindrical surface increases
 

Answera.T   b.T   c.T   d.T   e.T
 

9. The following are true about the spherical equivalent:

a. it is located at the circle of least confusion of the conoid of Sturm

b. it is midway between the dioptric powers of the two cylindrical lenses 
    that make up the astigmatic lens

c. it can be used to calculate if the eye is essentially emmetropic, 
    hypermetropic or myopic

d. it is used to choose intraocular lens during biometry

e. it is calculated by adding the spherical power to half the cylindrical power
 

Answera.T   b.F   c.T    d.T   e.T
The circle of least confusion of the conoid of Sturm is located at the focal 
point of the spherical equivalent lens, whose power is midway between the dioptric powers of the two cylindrical lenses that make up the astigmatic lens.
The spherical equivalent of a sphero-cylindrical lens can be calculated as follows:
Spherical equivalent = sphere + (cylinder/2)
It can be used to find out if an eye is emmetropic, hypermetropic or myopic. It is used in biometry to choose the intraocular lens.
. 
 
 

10. Jackson's cross cylinder:

a. does not blur the image when placed before an emmetropic eye

b. does not change the interval of Sturm according to the position of 
    the Sturm

c. does not alter the spherical equivalent of an ametropic eye

d. is used to check the axis of the cylinder subjectively

e. is used to check the power of the cylinder subjectively
 
 

Answera.F  b.F  c.T  d.T  e.T
Placed before an emmetropic eye, the cross-cylinder blurs the image. Placed before an ametropic eye, the cross-cylinder does not alter the spherical equivalent, but it will enlarge or contract the interval of Sturm, blurring or clarifying the image, as it increases or decreases the net astigmatic ametropia. The cross-cylinder is used for subjective refinement of axis and power of cylinder after placing the 
best available estimate of refraction before the eye (retinoscopy, astigmatic dial test, or previous refraction). 
 
 

11. The following are true about Jackson's cross cylinder:

a. it is used to check the axis of the cylinder before the power

b. the power of the cylinder is twice that of the sphere and of opposite sign

c. a 0.50D cross cylinder has a total cylindrical power of 0.50D

d. a 0.50D cross cylinder has a net spherical power or spherical equivalent 
    power of 0

e. a 0.50D cross cylinder can be written up as +0.25DS/-0.50DC
 
 

Answera.T   b.T   c.T   d.T   e.T

It is used to check the axis of the cylinder before its power. The power 

of the cylinder is twice that of the sphere and of opposite sign. A 0.50D 
cross cylinder has a total cylindrical power of 0.50D. A 0.50D cross cylinder
has a net spherical power or spherical equivalent power of 0. A 0.50D cross cylinder can be written up as +0.25DS/-0.50DC or -0.25DS/+0.50DC. Axis is not specified.