DISP - Ophthalmic Lens Forms - Week 3

Question 1

When does and doesn’t lens form become important when considering where images are formed with modern lenses (2)?

Answer 1

The lens form isnt important when looking through the optical axis as its performance here is excellent.
When images are formed from off-axis regions of a lens, the lens form is important to performance.

Question 2

Name 4 aberrations that adversely affect performance in off-axis regions.

Answer 2

Transverse chromatic aberrations
Oblique astigmatism
Field curvature
Distortion

Question 3

Describe two ways in which off-axis aberrations can be reduced via lens form.

Answer 3

Using the best form approach where curves are kept spherical.
Can also use an aspherical approach.

Question 4

Define chromatic aberration, and why it occurs.

Answer 4

When the lens fails to focus light of different wavelengths to a single point.
This is because lenses have different refractive indices for different wavelengths of light.

Question 5

How does the refractive index of a lens differ with the wavelength of light?

Answer 5

Refractive index decreases with increasing wavelength.

Question 6

Name and describe the two kinds of chromatic aberrations.

Answer 6

Axial/longitudinal - different wavelengths are focussed on the optical axis, but not at the focal point.
Transverse/lateral - different wavelengths are focussed at different positions in the focal plane.

Question 7

Consider transverse chromatic aberration. Distinguish between the effects of high and low contrast.

Answer 7

High contrast - the effect is to cause coloured fringes surrounding the image. The image itself is in focus.
Low contrast - colour fringing may not be noticed, but instead, the effect causes a reduction in acuity.

Question 8

What is an Abbé number, and what does a high/low number represent?

Answer 8

A number used to represent the amount of chromatic aberration for a given lens material.
A high Abbé number means less chromatic aberration.
A low Abbé number means its more likely that colour fringes will be observed.

Question 9

Describe the formula for Abbé number.

Answer 9

V = 1 / ω
V = Abbé number
ω = dispersive power
ω = (nF - nC) / (nD - 1)
nF = refractive index of blue light
nC = refractive index of red light
nD = refractive index of yellow light

Question 10

What lens material has the highest and lowest Abbé number?

Answer 10

Highest - crown glass

Lowest - polycarbonate

Question 11

How can the amount of transverse chromatic aberration at any given point be calculated?

Answer 11

Calculating the prismatic effect, and dividing by the Abbé number

Question 12

What threshold value for transverse chromatic aberration will typically cause problems?

Answer 12

~ 0.1Δ

Any less, and patients are unlikely to complain.

Question 13

If the Abbé number for normal index glass/plastic is ~60, what prism dioptre would be needed to reach the transverse chromatic aberration threshold?
What consequence does this have on low Abbé number lens materials?

Answer 13

p ~0.1 x V
and so prismatic effect would need to be 6Δ before it becomes noticed.The lower the Abbé number, the more likely the aberrations will be noticed.

Question 14

In oblique astigmatism, which focal line is more anterior (closer to the lens)?

Answer 14

Tangential is more anterior to the lens than the sagittal.

Question 15

Where are off-axis points brought into focus in field curvature?

Answer 15

The petzval surface.

Question 16

Consider distortion. What image shape does positive and negative distortion result in, and does it affect image clarity?

Answer 16

Doesnt affect image clarity.
Positive distortion (in positive lenses) results in a pincushion effect, negative distortion (in negative lenses) results in a barrel effect.

Question 17

In what kind of lens is distortion only a problem in practice?

Answer 17

High powered lenses.

Question 18

How can distortions be minimised?

Answer 18

By using aspheric lenses.

Question 19

Why does distortion occur?

Answer 19

Because of the differences in magnification between different parts of the lens.

Question 20

Describe the pincushion and barrel effect of distortion.

Answer 20

Pincushion - the periphery is magnified.

Barrel - the periphery is minified.

Question 21

In practice, what kinds of aberrations can a lens designer reduce (3)?

Answer 21

Can only reduce oblique astigmatism and field curvature, but not transverse chromatic aberrations as much.

Question 22

Describe the process of making a lens beginning with a blank (4).

Answer 22

A spectacle lens is made from a blank.
The front of the lens is surfaced to an appropriate base curve.
These blanks are held in storage until an order arrives.
Most appropriate base curve blank is assigned to an order, and the back curve is produced.

Question 23

Describe what point focal lenses are.

Answer 23

When oblique astigmatism is completely corrected, but the expense of having power error uncorrected.

Question 24

Describe what percival form lenses are. What kind of mean oblique error do they have?

Answer 24

When power error is corrected completely, at the expense of having residual oblique astigmatism uncorrected.
These lenses have zero mean oblique error.

Question 25

Describe what T-error lenses are.

Answer 25

Lens designed to have the minimum tangential error.

Question 26

Give three examples of best form lenses.

Answer 26

Point focal lenses
Percival form lenses
T-error lenses

Question 27

What is the most common form of best form lenses today and why?

Answer 27

Minimum T-error lenses, perform well over a wide range of vertex distances.

Question 28

Define aspheric surface.

Answer 28

A surface that is rotationally symmetrical, but at the same time, not spherical.

Question 29

Which is lighter and thinner, spheric or aspheric lenses?

Answer 29

Aspheric

Question 30

Does aspheric surfacing reduce aberrations in high or low powers? what does this come at the expense of?

Answer 30

Reduces aberrations in high plus power lenses, at the expense of a rapid decrease in power as the eye rotates away from the optical axis. This means there is some spherical error.