Astigmatism, subjective refraction & optometers

Question 1

Describe how astigmatism arises in the eye

Answer 1

*Caused by one or more of the refracting surfaces not being spherical, or by the surfaces being titled with respect to each other.

*Astigmatism of front surface of cornea is most common source.

*Back surface rarely has more than one dioptre of astigmatism.

• Lenticular astigmatism usually caused by tilting of crystalline lens rather than non-sphericity of the surfaces.

Question 2

Simple astigmatism

Answer 2

If one of the lines is on the retina

Question 3

Compound astigmatism

Answer 3

If neither lines are on the retina

Question 4

Myopic astigmatism

Answer 4

If the lines are in front of the retina

Question 5

Hypermetropic astigmatism

Answer 5

If the lines are behind the retina

Question 6

Mixed astigmatism

Answer 6

In special case where one line is in front and one behind the retina

Question 7

Direction of Principal meridians
WITH-THE-RULE

Answer 7

Axis of least powerful meridian of eyes optical system is no more than +/-30 degrees from the horizontal. If the axis of the negative correcting cylinder is under 30 degrees or over 150 degrees

Question 8

Direction of Principal meridians
AGAINST-THE-RULE

Answer 8

Least powerful meridian within 30 degrees of the vertical. In this type of astigmatism axis of the negative correcting cylinder is between 60 degrees and 120 degrees

Question 9

Direction of Principal meridians
OBLIQUE

Answer 9

When the axes of principal meridians fall outside these ranges that is when the axes are between 30 degrees and 60 degrees or between 120 degrees and 150 degrees

Question 10

What is meant by the term Subjective Refraction?

Answer 10

The determination of the refractive error using responses from the px.

Question 11

Description of the cross-cylinder

Answer 11

*Flat lens
*38mm diameter
*Two plano cyls of equal &
opposite powers
*UK NEGATIVE axis marked in
WHITE and POSITIVE marked in
RED
*Other countries use BLACK for
POSITIVE and RED FOR
NEGATIVE
*Handle at 10cm long
*Handle at 45 degrees to axes

Question 12

What are the most commonly used powers for a cross-cylinder lens?

Answer 12

+/-0.25DC or +/-0.50DC

Question 13

Astigmatic subject subject has been found to require a negative cylinder with its axis at 180. If the cross-cylinder is used to finalise the cylinder power what would be the orientation of the cross-cyl handle during this procedure?

Answer 13

Handle should be along 45 or 135

Question 14

Explain what is meant by a subjective optometer

Answer 14

An optometer is a device for measuring a subjects ametropia. Subjective optometers determine the ametropia using observations made by the subject

Question 15

Simple Optometer

Answer 15

*Consists of positive lens (+10.00D -+14.00D) placed in the spectacle plane

*Target is placed on far end of scale & gradually brought closer to the lens until subject reports it appears clear

*Image formed by the lens is now the subjects far point & vergence leaving optometer lens is equal to spec correction required at vertex distance

*Spec correction is read off from the scale

Question 16

Advantages of Badal Optometer

Answer 16

*Consists of high plus lens. It is placed so that its 2nd focal point lies in the spectacle plane

• Has a sliding target

*It’s moved along a scale until the subject reports that its clear & spec correction is read off the scale

Question 17

Disadvantages of the Simple Optometer

Answer 17

*Has a number of advantages

*Scale is non-linear being very compressed in the myopic range, makes reading the result less accurate

• No control over subjects accommodation which tends to be stimulated as target is brought closer which makes results too myopic

*Effect is increased by apparent increase in size of target as its brought closer