OV1004 Ophthalmic lenses and dispensing I

Question 1

What is the principle focus?

Answer 1

The point at which it focuses light from infinity

Question 2

Define focal length

Answer 2

Distance fro the vertex of the lens to the 2nd principle focal point

Question 3

What are the properties of a lens with a shorter focal length?

Answer 3

• More powerful
• Stronger lens
• Steeper curved lens

Question 4

What are the properties of a lens with a longer focal length?

Answer 4

• Less powerful
• Weaker lens
• Flatter curved lens

Question 5

What is the problem with hypermetropia?

Answer 5

Overall dimensions too short
Power too strong
Image falls behind the retina

Question 6

How do you correct a hypermetropic eye?

Answer 6

Using a positive spherical lens

Question 7

What is the problem with myopia?

Answer 7

Lens too strong
Eye is longer than it should be
Image falls in front of the retina

Question 8

How do you correct a myopic eye?

Answer 8

Using a negative lens

Question 9

What is the problem with astigmatism?

Answer 9

Optical surfaces of the eye are aspherical
Two principal powers are created (Maximum & Minimum power)
Two line foci are produced instead of dot image
Creates blurred image

Question 10

How do you correct an astigmatic eye?

Answer 10

Use a cylindrical lens

Two powers at perpendicular meridians

Question 11

How do you transpose a prescription?

Answer 11

• Add sphere and cyl lens together
• Change power of cyl
• Change axis by 90 degrees (subtract 90)