Astigmatic Lenses

Question 1

How are cylindrical lenses made

Answer 1

one plane surface and the other forms part of a cylinder. Thus, in one meridian the lens has no vergence power and this is called the axis of the cylinder

Question 2

In which meridian of a cylindrical axis does the cylindrical lens act as a spherical lens

Answer 2

In the meridian at right angles to the axis
The total effect is the formation of a line image of a point object. This is called the focal line.

Question 3

What is the focal line parallel to

Answer 3

It is parallel to the axis of the cylinder

Question 4

What is the Maddox rod used for

Answer 4

diagnosis of extraocular muscle imbalance, consists of a series of powerful convex cylindrical lenses mounted side by side in a trial lens

Question 5

How is the maddox rod test performed

Answer 5

The patient views a distant white point source of light through the Maddox rod, which is placed close to the eye (in the trial frame). The spotlight must be far enough away for its rays to be parallel on reaching the patient (at least 6 m). Light in the meridian parallel to the axis of each cylinder passes through undeviated and is brought to a focus by the eye

Question 6

What is the location of the line seen by the patient using the maddox rod

Answer 6

The line seen by the patient lies at 90° to the axis of the Maddox rod and is formed by the focusing mechanism of the eye. It is not the real line image of the Maddox rod

Question 7

How is the Maddox rod used to test muscle imbalance

Answer 7

Maddox rod is placed close in front of the right eye (in the trial frame) and the distant white spotlight is viewed with both eyes. The right eye therefore sees a red line, at 90° to the axis of the Maddox rod, while the left eye sees the white spotlight.

Question 8

How to test for a horizontal muscle imbalance using Maddox rod

Answer 8

To test for horizontal imbalance, the rod must be horizontal to give a vertical line and vice versa

Remember that the eye behind the Maddox rod (conventionally the right) is deviating in the opposite direction to that indicated by the red line

Question 9

Describe Maddox rod findings in horizontal orthophoria

Answer 9

Vertical line and rod intersect in the middle of patient view

Question 10

Describe Maddox rod findings in right eye Exophoria

Answer 10

Crossed diplopia
Vertical line formed to left of circle

Question 11

Describe Maddox rod findings in right eye Esophoria

Answer 11

Vertical line to right of circle

Question 12

Describe Maddox rod findings in vertical orthophoria

Answer 12

Horizontal line and circle intersect in the middle of patient view

Question 13

Describe Maddox rod findings in Right hyperphoria

Answer 13

Circle above horizontal line

Question 14

Describe Maddox rod findings in Right hypophoria (left hyperphoria)

Answer 14

Horizontal line above circle

Question 15

What is a toric surface

Answer 15

Imagine that the cylindrical lens is picked up by its ends and bent so that the axis XY becomes an arc of a circle. The previously cylindrical surface is now curved in both its vertical and horizontal meridians, but not to the same extent. It is now called a toric surface

Question 16

In a toric lens, what is the principal meridian of minimum curvature and therefore minimum power called

Answer 16

base curve

Question 17

What is a lens with one toric surface called

Answer 17

toric lens or sphero-cylindrical lens. For example a toric lens with a power of +2 D in one principal meridian and +4 D in the other principal meridian can be regarded as a +2 D sphere with a +2 D cylinder superimposed. This is therefore written as +2.0 DS/+2.0 DC

Question 18

How to calculate the spherical equivalent of a toric lens

Answer 18

algebraic addition of the spherical power and half the cylindrical power, e.g. the spherical equivalent of +2.00 DS/+2.00 DC is +3.00 DS, while that of +2.00 DS/–2.00 DC is +1.00 DS

The focal point of the spherical equivalent would coincide with the circle of least confusion of the toric lens’s Sturm’s conoid

Question 19

What is the jackson cross cylinder

Answer 19

a sphero-cylindrical lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign
The lens is mounted on a handle which is placed at 45° to the axes of the cylinders.

Question 20

What is the +0.50D cross cylinder used for

Answer 20

he 0.50 D cross-cylinder is used to check the power of the trial cylinder where the patient has good vision

Question 21

What is the +1.00D cross cylinder used for

Answer 21

The 1.00 D cross-cylinder is used to check the axis of the trial cylinder, and the power in patients with poor visual acuity

Question 22

Clinical use of the cross cylinder

Answer 22

check the axis of the cylinder prescribed and then its power. It can also be used to verify that no cylindrical correction is necessary for the patient if no cylinder was detected on retinoscopy.

Question 23

Which side of the JCC is plus axis and which is minus

Answer 23

White axis of plus cylinder

Red axis of minus cylinder