Session 7

Question 1

2 reasons when TO specify base curve

Answer 1

-when replacing one lens in a pair - so they match
-when ordering a identically powered pair that the px is going to wear interchangably

Question 2

4 reasons when NOT TO specify base curve

Answer 2

-Do not match to previous glasses - ideal bc for rx.
-Do not request a flatter base curve to get a thinner lens - causes peripheral aberrations.
-Do not change the base curve to solve ghost image internal lens reflections- ar coating used instead.
-do not steepen the base curve to accommodate long eyelashes- introduce unwanted aberrations

Question 3

What are the 2 categories of aberrations?

Answer 3

Chromatic Aberrations and Monochromatic Aberrations

Question 4

What is Chromatic Aberration?

Answer 4

White light is broken into its spectral components and observed as colour fringes

Question 5

What is Monochromatic Aberration?

Answer 5

Occurs when the light entering the lens is only one colour.

Question 6

What is distortion?

Answer 6

aberration that causes objects to appear in other than their true shape

Question 7

What are two types of distortion? and the causes?

Answer 7

Pin cushion effect - high plus
Barrell shape - high minus

Question 8

How do you fix distortion?

Answer 8

Choose an aspheric as the edges will be flatter.

Question 9

What is latitudinal/transverse chromatic aberration?

Answer 9

Produces images of different sizes at the focal length of the lens

Question 10

What light colour bends the most/least?

Answer 10

Red bends the least. Blue bends the most.

Question 11

What is longitudinal/axial chromatic aberration?

Answer 11

When a point light source that is composed of several wavelengths forms a series of point images along the optical axis.
Each image is a diff colour and diff focal length

Question 12

How is refractive index and abbe value related

Answer 12

The higher the refractive index the lower the abbe value.
The lower the abbe value the more likely it will be that chromatic aberration will occur

Question 13

what is abbe value?

Answer 13

a number to identify the amount of chromatic aberration for a lens material

Question 14

What two types of aberrations can Opticians fix?

Answer 14

Chromatic aberration
Distortion

Question 15

What are the 5 types of Monochromatic aberration?

Answer 15

Spherical Aberrations
Coma
Marginal/Oblique Astigmatism
Curvature of Field
Distortion

Question 16

What is spherical aberrations and what fixes it?

Answer 16

When peripheral rays focus at different points on the optic axis compared to the para-axial (more central).

Small pupil limits the amount of rays that get through to the eye so not a problem in glasses

Question 17

What is Coma Aberrations and what fixes it?

Answer 17

The image sizes viewed are different due to light being displaced. Looks Cometlike fashion.

Small pupil limits the amount of rays that get through to the eye so not a problem in glasses.

Question 18

What is Marginal/Oblique astigmatism aberrations and what can fix it?

Answer 18

When light hits a lens at a off axis and causes two focus points.

Find the optimum Base Curve, it can be reduced but never minimized.

Question 19

What is curvature of field aberration and what fixes it?

Answer 19

Looking through periphery of the lens causes off power effect

Find the optimum Base Curve, it can be reduced but never minimized.

Question 20

How do you reduce chromatic aberration?

Answer 20

Choose a smaller frame rather than higher refractive index where possible