6 - Corrective Curve Lens Design - Exam 2

Question 1

What dioptric power does the back surface of a periscopic lens have?

Answer 1

-1.25 D

Question 2

What dioptric power does the front surface of a meniscus lens have?

Answer 2

+6.00 D

Question 3

Which aberration were the first corrective curve lenses designed to minimize?

Answer 3

Radial astigmatism

Question 4

What is true about base curves that correct or minimize one type of aberration?

Answer 4

They often make another type of aberration worse

Question 5

Would a patient with high sensitivity to chromatic aberrations be more receptive to a high or low abbe value material?

Answer 5

High abbe value

Chromatic aberrations minimized

Question 6

What are the 5 types of Seidel (third-order) aberrations?

Answer 6

1. Spherical aberration
2. Coma
3. Radial astigmatism (AKA oblique astigmatism)
4. Curvature of field
5. Distortion

Question 7

Which 2 types of third-order aberrations are minimized by the pupil?

Answer 7

1. Spherical aberrations

2. Coma

Question 8

Which 2 types of third-order aberrations are minimized by selecting the appropriate base curve?

Answer 8

1. Radial astigmatism

2. Curvature of field

Question 9

Which type of third-order aberration can not be eliminated, but also is the least problematic?

Answer 9

Distortion

Question 10

What are the 2 most important types of off-axis aberrations to control?

Answer 10

1. Radial astigmatism

2. Curvature of field

Question 11

Where is a hyperope’s far point located?

Answer 11

Behind the eye

Question 12

What is the far point sphere?

Answer 12

The locus of far points for all possible viewing angles

Question 13

For radial astigmatism, will the tangential focus or the sagittal focus be closer to the lens?

Answer 13

Tangential focus

Effective tangential power > Effective sagittal power

Question 14

How many mm should you drop the optical center for 2 degrees of pantoscopic tilt?

Answer 14

1 mm

Question 15

If you drop the optical center 1 mm, how many degrees of pantoscopic tilt are you accounting for?

Answer 15

2 degrees

Question 16

In what type of eyewear are the effects of radial astigmatism and induced prism especially apparent?

Answer 16

Wrap-around prescription eyewear

Question 17

What are 2 other names for curvature of field error?

Answer 17

1. Petzval surface

2. Power error

Question 18

How does the curvature of the Petzval surface compare to the curvature of the far point sphere

Answer 18

Petzval surface is usually flatter

Question 19

What type of lens corrects radial astigmatism completely but leaves the curvature of field uncorrected?

Answer 19

Point focal lens

Question 20

What type of lens minimizes the power error but leaves residual radial astigmatism?

Answer 20

Percival form lens

Question 21

What type of lens partially corrects both curvature of field error and radial astigmatism?

Answer 21

Minimum tangential error form lens

Question 22

Which type of corrective curve (or best form) lens is most common?

Answer 22

Minimum tangential error form lens

Question 23

What are 4 important parameters to consider with lens design?

Answer 23

1. Vertex distance
2. Lens thickness
3. Refractive index
4. Front and back lens surface powers

Question 24

What is the purpose of a Tscherning ellipse?

Answer 24

Tell what base curves eliminate radial astigmatism for a given lens power

Question 25

Where does the Tscherning ellipse not cover, and what does this mean?

Answer 25

Does not extend into high plus power range | Radial astigmatism can not be eliminated for aphakes or high hyperopes

Question 26

How is Tscherning's ellipse used in the real world?

Answer 26

Divided into sets of prescriptions for a given base curve (+10 BC for high plus, +2 BC for high minus, etc.)

Question 27

What base curve does a plano lens usually have?

Answer 27

+6.00 D

Question 28

How does the back surface of a lens change as you increase minus power?

Answer 28

Back surface gets steeper with more minus

Question 29

How does the back surface of a lens change as you increase plus power?

Answer 29

Back surface gets flatter with more plus

Question 30

What is Vogel's formula for a plus lens?

Answer 30

BC = Spherical Equivalent + 6.00 D

Question 31

What is Vogel's formula for a minus lens?

Answer 31

BC = Half of the spherical Equivalent + 6.00 D

Question 32

For different powered left and right lenses, which base curve should you choose if both lenses are plus?

Answer 32

The steeper base curve

Question 33

For different powered left and right lenses, which base curve should you choose if both lenses are minus?

Answer 33

The flatter base curve

Question 34

For different powered left and right lenses, which base curve should you choose if one lens is plus and the other is minus?

Answer 34

The base curve with the higher numerical value (steeper)

Question 35

What base curve are most frames designed to be used with?

Answer 35

+6.00 D

Question 36

How may plus lenses need to be altered to fit in metal frames?

Answer 36

They may need to be flattened

Question 37

How may minus lenses need to be altered to fit in metal frames?

Answer 37

They may need to be steepened

Question 38

What is the effect of prism on magnification of a lens?

Answer 38

Increases magnification | Due to an increase in lens thickness

Question 39

What may be used to reduce prismatic effects?

Answer 39

A flatter base curve