7 Aberrations & Lens Design

T or F: a theoretically perfect optical surface would still have aberrations

T

Aberrations are caused by

• the way light is refracted by a surface

• index of medium

• location of object point

Chromatic aberrations are dependent on

lens material

monochromatic aberrations are dependent on

lens design (form)

For most materials, the refractive index is higher for (short or long) wavelengths

short (blue)

Abbe value equation with blue and red

Longitudinal Chromatic Aberration (LCA)

dioptric distance between red and blue focus = P/V

• aka axial CA

What clinical test makes use of eye’s longitudinal chromatic aberration?

Duochrome (red/green) test

Transverse Chromatic Aberration (TCA)

• aka lateral chromatic aberration

• differences in prismatic effects (angular dispersion) of lens for various wavelengths

What would be the TCA at the optical center?

0

What aberration causes colored fringes around borders and blurs image?

TCA

How can we control chromatic aberration?

achromatic doubleta

Achromatic doublet

controls chromatic aberration by having a plus and minus lens together (lenses of different Abbe Values)

• not a practical design for spectacle lenses

How much of an issue are chromatic aberrations with spectacle lenses?

not significant, there are ways to control

Ways to control chromatic aberration

• use material with higher Abbe Value (trivex > polycarb)

• center pupil in frame (Frame PD = Patient PD, and centered vertically) → no decentration decreases lens thickness and chance of induced prism → less TCA

• use shorter VD

If we could limit light passing through the lens to one wavelength (monochromatic light), what aberration would be eliminated?

chromatic aberrations

Monochromatic aberrations: The Seidel Aberrations

S1 Spherical aberration

S2 Coma

S3 Oblique Astigmatism

S4 Curvature of Field

S5 Distortion

Which Seidel Aberrations cause the most problems with spectacle lenses?

Oblique Astigmatism and Curvature of Field

Spherical aberration

• blur when a pencil of light [parallel to optic axis] is refracted by a large-aperture optical system

• occurs because different zones of the aperture have different focal lengths

Positive spherical aberration is when peripheral lens zones have (shorter or longer?) focal lengths than paraxial zones

shorter (=periphery more powerful)

Which is more common, positive or negative spherical aberration?

positive

Longitudinal (axials) spherical aberration is

the dioptric separation of foci of paraxial and peripheral zones

Lateral (transverse) SA is

linear diameter of image formed (called confusion disc) on a screen at any point between paraxial and peripheral foci

Making the entrance pupil (smaller/larger) would help reduce spherical aberration

smaller

Bottom line: spherical aberration is of minor significance with moderate lens power, and is only considered for high powered (concave/convex?) lenses

convex, for which plastic aspheric lenses are routinely used

Coma occurs when

oblique rays are refracted by a large aperture optical system

• affects sharpness of image points

Resulting image of coma is __-shaped

teardrop or comet

Which is the least important monochromatic aberration in spectacle design?

coma

Oblique astigmatism aka

• radial

• marginal

• off-axis

• peripheral

oblique astigmatism (the aberration) occurs when

narrow pencil of light from an object passes obliquely through a spherical surface

• rays of light form interval of Sturm

Interval of Sturm

occurs with oblique astigmatism, made of 2 line foci and a COLC

The two line images formed with oblique astigmatism are called

Tangential and Sagittal Focus/Image

Tangential Plane contains the chief way from the object point and optic axis

the object point and optic axis

Sagittal plane contains chief ray from

the object point, and is perpendicular to tangential plane

The dioptric separation of the T and S images is the amount of

oblique astigmatism

The goal of spec lens design is to bring the tangential and sagittal planes

as close together as possible

Petzval surface

spherical image surface that touches the two focal surfaces at their common axial point - Google

When oblique astigmatism is present, a true _ surface does not exist, and the T and S foci fall on the same side of that surface

Petzval

The curvature of the tangential surface is always (flatter/steeper) than the sagittal

steeper

A Point Focal lens

spherical lens with no oblique astiematism

Features of a Point Focal lens

• CoR is 27 mm behind lens

• n = 1.523

  • power is between -23D and +7.25D

How many solutions are there for each lens power in Jalie’s equation?

2 (front curves for each power that give no oblique astigmatism)

Wollaston and Ostwalt

branches of Tscherning Ellipse, each are one solution of front curve needed to eliminate oblique astig in a given lens power

Which branch of Tschernings Ellipse is flatter and more cosmetically appealing?

Ostwalt

Which branch of Tschernings Ellipse minimizes distrotion?

Wollaston

When is there no curvature of image?

If the far point sphere coincides with the Petzval surface

How can we control curvature of image?

adjust base curve

What aberration must exist if curvature of image is corrected for?

oblique astigmatism

T of F: Distortion causes blur

F, just makes the image different from the shape of the original object

What is it called when the ratio of image size to object size has a constant value

orthoscopy (no distortion)

distortion is present when ratio of

image size to object size varies for diff obj sizes

Barrel Distortion

image size : obj size ratio decreases with increase in obj size

(with a minus lens, periphery has more minification than center)

Pincushion distortion

image size : object size ratio increases with increase in obj size

Plus lenses give what kind of distortion?

pincushion

minus lenses give what kind of distortion?

barrel

Distortion is mainly a problem for (low/high) powered lenses

high

Distortion can be minimized using

steep back surfaces

Potential Lens Design Variables

1. Number of lenses

2. n

3. Lens thickness

4. VD

5. Lens form

What lens design variable is usually used to minimize aberrations?

Lens Form

Chart to summarize how we control aberrations

What are the generic terms for lenses that correct for oblique astig and curvature of image?

Corrected Curve and Best Form

Point-focal lenses

Lens design that concentrates on oblique astig and ignores curvature of image

Oblique astigmatism is an __ error

astigmatic

Curvature of image is a __ error

spherical

Lens Design Assumptions

1. The center of rotation of the eye is a fixed point with respect to the cornea (not true, there’s a cloud of locations=body centrode)

2. The optic axis of the correcting lens passes through the center of rotation of the eye (relatively true, with minimal tilt)

3. The foveal chief ray passes through the center of rotation of the eye (almost, the fovea is slightly temporal)

Minus cyl design advantages

• less meridional mag

• better cosmetic appearance

How is base curve chosen?

Table (now computer programs) of sphere and cyl powers → base curve: Compromise between using single base curve for all lenses (and grinding the needed power on back surface) and using different base curve for each lens power (using Tscherning Ellipse)

High plus lenses would theoretically have very (flat/steep) base curves, causing several problems…so we use aspheric lens designs

steep

Functional and cosmetic problems of steep base curve for high plus lenses

Thicker/heavier lens

More magnification (from thickness and increased VD)

Cosmetically not pleasing

Why are aspheric lens designs used?

To control oblique astig in lenses +7.50 (see problems with steep base curves with high plus lenses)

Advantages of aspheric design

• Correct off axis astig

• Thinner → better cosmetic appearance

• reduced mag

Disadvantages of aspheric design

• Off axis power errors from improper fitting are greater than for spherical lenses (blur from not looking through correct power)

• Cannot use prism by decentration (they would be looking through the wrong power)

• More expensive than spherical lenses

For what powers should aspheric lenses be considered?

> +- 4.00