17 - Aberration

Question 1

What are 3 characteristics of ‘flat’ lenses?

Answer 1

-1200AD-1800AD
-bean shaped
-good for central vision, bad for peripheral vision

Question 2

What are 2 characteristics of Periscopic Lenses?

Answer 2

-1800 AD onwards
-Improvement in peripheral vision when -1.25D back surface was used

Question 3

What are 3 characteristics of six-base meniscus lenses?

Answer 3

-Beginning of 1890
-peripheral vision increased alot more
-could be fit closer to eye, because the lens cleared the lashes.

Question 4

What are 2 characteristics of corrected curve lenses? (Carl Zeiss)

Answer 4

-1908 introduced Punktal lenses that correct oblique astigmatism in the peripheral
-require a very large number of base curves (lots of BCs correcting, complicated)

Question 5

What are 2 characteristics of corrected curve lenses? (American Optical)

Answer 5

-1919 American Optical also introduced lenses that correct oblique astigmatism in the peripheral
-were designed with base curves that changed in 1 or 2 dioptres intervals.(simplified)

Question 6

What are 3 characteristics of aspheric lenses (history) ?

Answer 6

-Available for very high plus ‘cataract’ style corrections
-beginning of 20th century
-were used more wide spread when higher index plastic lens materials become available

Question 7

What are 3 characteristics of atoric lenses (history) ?

Answer 7

-replaced aspheric
-used in progressives that are individual designed and custom produced by free-form techniques

Question 8

What is a lenticular lens? 3

Answer 8

Central vision has the prescribed power - aperture
peripheral is little or no power - carrier
Looks like a fried egg

Question 9

What is the Zernike Order of Aberrations? 5

Answer 9

1st - Prism
2nd - Defocus (refractive error -myopia and hyperopia) and Astigmatism
3rd - Coma and Trefoil
4th - Spherical Aberration and other Modes
5th - Higher Order Irregular Aberrations.

Question 10

What is Chromatic Aberration? 2

Answer 10

It causes an image to have a coloured fringe.
Light from a point source goes through a correctly powered lens yet does not create a perfect image.

Question 11

What are the 2 manifestations of chromatic aberrations?

Answer 11

Longitudinal
Lateral

Question 12

What are the effects of chromatic aberration on the vision?

Answer 12

• looking through OC = no prismatic effect/chromatic aberration
  -looking away from OC= prismatic effect increases so does chromatic power which blurs the image.

Question 13

What is Monochromatic aberrations?

Answer 13

• less of concern with glasses lenses, mainly problem in cameras
• aberrations occur when the light source contains only one wavelength (1 colour)

Question 14

What are the 5 Seidel Aberrations? 5

Answer 14

1-Spherical Aberration
2-Coma
3-Oblique Astigmatism
4-Curvature of Field
5-Distortion

Question 15

What are 3 general characteristics of Seidel Aberrations?

Answer 15

-know as seidel or 3rd order aberrations
-5 are interrelated
-changing the lenses to help one can affect the others

Question 16

What is spherical aberration? 3

Answer 16

-Smaller Pupil size helps correct, not an issue in spec lenses
-light enters a large lens surface, as the peripheral parts the light bends more making the rays came to a focus at a shorter distance
-rays come to focus at different points along the optic axis

Question 17

What is Coma? 3

Answer 17

-Smaller Pupil size helps correct, not an issue in spec lenses
-causes light from peripheral areas of the lens to focus further away from the image point it should be.
-the further away in the peripheral = displaced further away (comet like/ ice cream cone).

Question 18

What is Curvature of Field (Power Error)? 3

Answer 18

-spherical power of lens has the effect of being off power in the peripheral
-More troublesome in ophthalmic lenses
-optimum base curve will help

Question 19

What is distortion? 2

Answer 19

-different magnification in different peripheral areas of the lens in proportion to the distance of these areas from the OC of the lens.
-More troublesome in ophthalmic lenses

Question 20

What happens to the image in a plus lens during distortion?

Answer 20

Pincushion distortion

Question 21

What happens to the image in a minus lens during distortion?

Answer 21

Barrell Distortion

Question 22

What is Oblique Astigmatism (Aberrations)? 5

Answer 22

-More troublesome in ophthalmic lenses
-optimum base curve will help
-occurs when rays from of-axis point pass through spec lens
-light focuses at 2 line images rather than a single point.
-when expressed in D this difference is called oblique astigmatic error

Question 23

What is the Abbe Value?

Answer 23

This is a number used to identify the amount of chromatic aberration for a material.

Question 24

What is the relationship between Abbe Value and Chromatic Aberration?

Answer 24

The higher the Abbe Value, the less chromatic aberration present. The lower the Abbe Value, the more likely colour fringes will be seen and VAs will be reduced in the periphery of high-powered lenses.

Question 25

List these materials from highest to lowest Abbe Value.
Trivex, 1.6 index, CR-39, Polycarbonate, Crown Glass

Answer 25

Crown Glass
CR-39
Trivex
1.6 Index
Polycarbonate

Question 26

What is a Achromatic Lens? 5

Answer 26

-a lens with no chromatic aberration = all the wavelengths across the visible spectrum focus at one point.
-does not happen with ophthalmic lens materials
-Not normally used in Spec lenses
-Combinations of 2 materials of different materials
-2 different refractive index’s to counteract the effects of chromatic aberration

Question 27

What is Longitudinal Aberration? 2

Answer 27

-light refracts at slightly different degrees of refraction causing different focus points along optical axis.
-slightly different colors

Question 28

What is Lateral Aberration?

Answer 28

-also known as chromatic power
-the difference in image magnification (different sizes)
-comes to a focus at the focal length of the lens.