astigmatism

Question 1

Define astigmatism

Answer 1

• A condition of refraction in which the image of a point object is not a single point but two focal lines at different distances from the optical system”
• “The two focal lines are generally perpendicular to each other” (‘regular astigmatism’)

Question 2

what can be a cause of astigmatism

Answer 2

toroidal surfaces

Question 3

Oblique astigmatism (1st meaning)

Answer 3

• If light from a point object passes obliquely through a spherical surface (or lens), then it will not form a point image.
• The point object is well below the principal axis
• In fact, it will give rise to the same imagery we get in astigmatism that is caused by a toroidal surface
• If the crystalline lens in the eye is tilted relative to the cornea, we will get oblique astigmatism, even if all the surfaces are spherical
• A much less common cause of astigmatism than toric surfaces

Question 4

impact of astigmatism on distance vision

Answer 4

• Like uncorrected myopia, uncorrected astigmatism always reduces distance vision
• less damaging than myopia however
• Oblique astigmatism is more damaging to vision compared to astigmatism where principal meridians are H&V, or near to H&V

Question 5

what are the different ways we can classify or describe astigmatism

Answer 5

• regular/irregular
• compound, simple, mixed
• with the rule, against the rule and oblique

Question 6

Define irregular astigmatism

Answer 6

• ocular astigmatism in which the two principal meridians are not at right angles to each other
• The condition is often the result of injury or disease (e.g. keratoconus), but can also exist in an eye with irregularities in the refractive power in different meridian of the crystalline lens
• cannot use normal sph/cyl as their principal meridians are at right angles so only for correcting regular astigmatism
• if cause of irregular astigmatism is corneal a contact lens is better for protection

Question 7

with-the-rule astigmatism

Answer 7

• refractive power of vertical meridian is the greatest
• corrected with -ve cyls with axis at 180 or near 180 OR positive cyls with axis at 90 or near 90

Question 8

Against-the-rule astigmatism

Answer 8

The refractive power of the horizontal (or near horizontal) meridian is the greatest
- corrected with -ve cyls with axis at 90 or near 90 OR with +ve cyls with axis at 180 or near 180

Question 9

oblique astigmatism (2nd meaning)

Answer 9

• in which the two principal meridians are neither approximately horizontal nor approximately vertical
• Oblique astigmatism is corrected with cyls with axis at 45 or 135, or near 45 or 135
• It doesn’t matter if the cyl power is +ve or –ve, if the cyl axis is 45 or 135 the eye has oblique astigmatism
• oblique astigmatism is less common than against the rule astigmatism and much less common than with the rule astigmatism

Question 10

Accommodation

Answer 10

• increases the power of the eye
• hyperopes benefit more since they are underpowered
• Accommodation shifts the “interval of sturm” leftwards so eye may be able to move the CLC onto the retina