Defects of Vision

Question 1

Myopia - short or long?

Answer 1

Short-sightedness

Question 2

Hypermetropia - short or long?

Answer 2

Long-sightedness

Question 3

What does it mean to be myopic?

Answer 3

Unable to focus on distant objects

Question 4

Why are people myopic?

Answer 4

Because their far point is closer than infinity

Question 5

When does myopia occur?

Answer 5

When the cornea and lens are too powerful or the eyeball is too long or the cornea is too curved.
The focusing system is too powerful and images of distant objects are brought into focus in front of the retina.

Question 6

What lens is needed to correct myopia?

Answer 6

A lens of negative power to reduce the overall power of the system. A diverging lens is therefore used to correct this defect.

Question 7

When does hypermetropia occur?

Answer 7

When the cornea and lens are too weak or the eyeball is too short.
The focusing system is too weak and images of near objects are brought into focus behind the retina.

Question 8

What lens is needed to correct hypermetropia?

Answer 8

A lens of positive power is needed to correct the defect, to increase the overall power of the system. A converging lens is therefore used to correct this defect.

Question 9

What is considered to be an acceptable near point?

Answer 9

0.25m

Question 10

What is astigmatism caused by?

Answer 10

An irregularly shaped cornea or lens which has different focal lengths for different planes.

Question 11

How is astigmatism corrected?

Answer 11

Using a cylindrical lens that adds power in one plane, but not the plane perpendicular to it.

Question 12

What does the prescription for the cylindrical lens to correct astigmatism state?

Answer 12

• The power needed to correct long or short sightedness
• The additional power needed to correct the astigmatism
• The angle to the horizontal of the plane that does not need correcting for astigmatism