Ch. 18: Lens Design

Power Error

"Curvature of Field" is also called:

Difference in Magnification Difference in Prismatic Effect

2 Ways by which the effect of Lateral Chromatic Aberration can be described:

Radial Astigmatism Marginal Astigmatism

2 other names for Oblique Astigmatism:

Longitudinal chromatic aberration Lateral (Transverse) chromatic aberration

2 types of Chromatic aberration:

Retinal/ocular imaging Refractive surgery Correcting aberrations in the eye

3 situations/reasons in which using the Zernicke polynomial model of monochromatic aberrations would be preferred over Seidel aberrations:

1. Optically correcting lens aberrations 2. Allowing lenses to be made flatter without increasing peripheral aberration 3. Making lenses thinner & more lightweight 4. Ensuring good, tight fit in the frame 5. Making lenses with progressive optics

5 main purposes for making lenses with an aspheric design:

Point Focal Lens

A lens design that completely corrects oblique astigmatism but leaves power error uncorrected is called:

Percival Form Lens

A lens design that completely corrects power error but leaves oblique astigmatism uncorrected is called:

Minimum Tangential Error Form Lens

A lens design that corrects both power error and oblique astigmatism to some extent (a compromise) is called:

Index of refraction (n)

A lens material actually has a different ___________________________________ for each wavelength of light.

+7.00 to -23.00D

According to Tscherning's ellipse, it is possible to completely correct oblique astigmatism for a sphere power range of:

Nu value Constringence

An Abbe value is also known as:

Zernicke polynomials

An alternative system to describing monochromatic aberrations is the system of:

Higher

An error in base curve selection is more problematic for _____________________-powered lenses.

Center of the lens surface

Aspheric lenses have a changing radius of curvature and therefore a varying amount of surface astigmatism, everywhere except:

2.00 D

Atoric lenses should be recommended for all cylinder powers above:

High

Crystal chandeliers are made to have a ___________ amount of chromatic aberration.

Selecting the appropriate base curve for a lens.

Curvature of Field can be controlled by:

Corrects oblique astigmatism in the lens periphery.

Describe the general improvements on optics in Corrected Curve lenses:

Base out; Base in

Face-form lenses induce _______________ prism, and therefore must incorporate ___________________ prism in the Rx to neutralize the effect.

Central vision Peripheral vision

Failure to select the correct base curve for a given lens power will NOT affect _________________ quality, but it will degrade the quality of ________________.

6.00D

For a plano lens, the back surface curve is usually:

Front & Back lens surface powers

For most single-vision "stock" lenses, most lens design variables are pre-determined. Which variable is left to make adjustments for optimal use for the patient's Rx?

Front surface = spherical Back surface = spherical

For spherical (simple sphere) lenses, what is the construction/design of the front and back curves?

Front surface = spherical Back surface = toric

For spherical spherocyl lenses, what is the construction/design of the front and back curves?

Flatter; steeper

From the front, minus lenses appear __________________ and plus lenses appear _________________.

MUCH Flatter

High-index lens base curves are usually ___________________ than what is calculated by Vogel's formula.

Tilting a lens in front of the eye

How can oblique astigmatism be induced using a simple spherical or spherocylinder lens?

Finding the optimum base curve for a given lens power

How may Oblique Astigmatism in a lens be reduced?

MORE

If a lens material has a LOWER Abbe value, it will have _______________ (more/less) chromatic aberration:

Down

If light enters a lens from ABOVE the optical axis, what is the base direction of the induced prism?

Up

If light enters a lens from BELOW the optical axis, what is the base direction of the induced prism?

Optical axis

In Coma, the "tail" of the comet-shaped image points toward the:

Where the lens SHOULD focus light

In Curvature of Field aberration, the Far Point Sphere is:

Where light actually focuses due to the aberration

In Curvature of Field aberration, the Petzval surface is:

Image Shell Error

In Curvature of Field aberration, the dioptric difference between the far point sphere and the Petzval surface is called:

Longer

In Lateral chromatic aberration, ___________________________ (longer/shorter) wavelengths create larger images.

Low

Lens materials with _______________ (low/high) chromatic aberration have nearly the same n-value at all wavelengths.

High

Lens materials with _________________ (low/high) chromatic aberration have a wide range of n-values for different wavelengths.

Barrel

Minus lenses create which kind of distortion?

+6.00

Most metal frames are made to best fit a lens with a base curve of:

Spherical aberration Coma Oblique Astigmatism Curvature of Field Distortion

Name the 5 types of Seidel Aberrations:

Aspheric

Outside of the sphere power range of +7.00 to -23.00D, a(n) ____________________ lens design is required to completely correct oblique astigmatism.

0 < p < 1

P-value of a Prolate ellipse aspheric surface?

p = 1

P-value of a circle aspheric surface?

p<0

P-value of a hyperbola aspheric surface?

p = 0

P-value of a parabola aspheric surface?

p >1

P-value of an oblate ellipse aspheric surface?

Flatter

Plus lens base curves are actually ___________________ than what is calculated with Vogel's formula.

Pincushion

Plus lenses create which kind of distortion?

Low Abbe values

The "drop-off" in visual acuity when looking away from the OC of a high-powered lens occurs faster in lenses with:

High-powered

The "drop-off" in visual acuity when looking away from the OC of a lens occurs faster in ______________ lenses.

Chromatic aberration Monochromatic aberration (Seidel aberrations)

The 2 main categories of aberration:

Oblique astigmatism Curvature of Field Distortion

The 3 Seidel aberrations that are the most problematic in ophthalmic lenses are:

Point Focal lens Percival Form Lens Minimum Tangential Error Form lens

The 3 types of Corrected Curve (or "Best Form") lenses:

Petzval surface

The ________________________ is formed when oblique astigmatism is corrected.

Abbe value

The ________________________ of a lens material describes the amount of chromatic aberration in a given lens material.

Using a flatter base curve

The magnification effect that comes with high amounts of Rx prism may be reduced by:

Yellow

The n-values we normally use for lenses are based on what color of light?

Longitudinal chromatic aberration

Type of aberration that occurs when rays of different wavelengths focus at different points along the optical axis:

Lateral chromatic aberration

Type of aberration that occurs when rays of different wavelengths produce images of different sizes:

Use monocular PD's Measure MRP heights including pantoscopic angle Have an appropriate amount of pantoscopic tilt

Ways to minimize aberrations when dispensing lenses with low Abbe values:

Circle Ellipse Parabola Hyperbola

What are the 4 types of conic sections used to make aspheric lenses?

Vertex Distance Lens Thickness Refractive Index Front & Back lens surface powers

What are the 4 variables of lens design?

Defocus (myopia/hyperopia) Astigmatism

What are the aberrations seen in 2nd-order Zernicke polynomials?

Coma Trefoil

What are the aberrations seen in 3rd-order Zernicke polynomials?

Spherical aberration & other modes

What are the aberrations seen in 4th-order Zernicke polynomials?

Higher order irregular aberrations

What are the aberrations seen in 5th through 10th-order Zernicke polynomials?

Lower curve of Tscherning's ellipse

What graph is currently used to determine base curves for select lens powers?

Prism

What is the aberration seen in first-order Zernicke polynomials?

Zernicke doesn't assume that the eye is composed of all spherical surfaces

What is the advantage of using Zernicke polynomials instead of the Seidel aberrations?

The vertical meridian

When adding face-form tilt to a lens (along the 90), which meridian is the sagittal meridian?

The horizontal meridian

When adding pantoscopic tilt to a lens (along the 180), which meridian is the sagittal meridian?

When using high-index materials or fused multifocal segments on glass lenses

When designing a spectacle lens, for what situations is it important to consider the effect of Chromatic aberrations?

Conic section

When designing aspheric lenses, the surface curvature is based on a ___________________________.

Flatter base curve of the 2 should be chosen.

When the 2 recommended base curves for a pair of lenses are different and both lenses are MINUS, how should you choose which base curve to use?

Steeper (higher numerical) base curve should be chosen (*but may not be the best cosmetic choice*)

When the 2 recommended base curves for a pair of lenses are different and both lenses are PLUS, how should you choose which base curve to use?

Choose base curve of higher numerical value

When the 2 recommended base curves for a pair of lenses are different and one lens is plus and the other minus, how should you choose which base curve to use?

The same as the axis of tilt

When tilting a lens, induced cylinder axis is:

The same

When tilting a lens, induced cylinder power is _____________________ (the same/opposite of) the sign of the sphere power of the lens.

Spherical aberration Coma

Which 2 Seidel aberrations' effects are limited by the pupil?

Crown Glass/CR-39

Which 2 commonly used lens materials have a particularly high Abbe value?

Spherical aberration

Which Seidel aberration occurs ON the optical axis?

Polycarbonate

Which commonly used lens material has a particularly low Abbe value?

Circle

Which conic section type is formed by a horizontal slice through an upright cone?

Hyperbola

Which conic section type is formed by a plane that makes a greater angle with the base of the cone makes with the base?

Ellipse

Which conic section type is formed by an angled slice through a cone that doesn't intersect the base?

Parabola

Which conic section type is formed by the intersection of a cone with a plane having 1 side parallel to the side of the cone?

Atoric

Which lens design optimizes each meridian of astigmatism correction on a toric lens?

Achromatic Doublet lenses

Which lens type is specifically made to eliminate chromatic aberrations?

1st and 2nd order

Which orders of Zernicke aberrations can be corrected with a glasses prescription?

Coma

Which type of aberration occurs when an off-axis object has a difference of magnification at different zones of the lens, creating a comet-shaped image?

Curvature of Field

Which type of aberration occurs when light entering peripheral areas of a lens does not focus on the far point sphere?

Oblique Astigmatism

Which type of aberration occurs when off-axis rays create 2 line images, called the tangential & sagittal images?

Spherical aberration

Which type of aberration occurs when parallel light enters a large area of a spherical lens surface, and peripheral rays focus at different points on the optical axis than paraxial rays?

Distortion

Which type of aberration occurs when there is different magnification at different areas of the periphery of a lens?

Steepens; flattens

With respect to base curve, as lens power becomes more minus, the back surface ________________ and the front surface ________________.

Flattens; steepens

With respect to base curve, as lens power becomes more plus, the back surface ________________ and the front surface ________________.

Plano

___________ Prisms do NOT have longitudinal chromatic aberration.

Achromatic Doublet

______________________________ lenses are created with a plus lens of one material in combination with a minus lens of another material.