7.3 Lens Design

Why is a single lens usually used as opposed to a doublet?

weight and appearance

List the potentially lens design variables. Which one is actually used to affect aberrations?

1. number of lenses

2. index of refraction

3. lens thickness

4. vertex distance

5. lens form

Why is index of refraction not a lens design variable used to control aberrations?

index of refraction is standard for a given material

Why is lens thickness not a variable used to control aberrations?

safety and appearance

When is lens thickness varied for optics?

RIS in aniso patients

Why is vertex distance not a variable used to control aberrations?

too limited:

-long vertex distance will decrease field of view

-short vertex distance is blocked by face

How many variables do we have to control lens aberrations?

only one

How is spherical aberration eliminated?

aspheric design (usually already used in high plus lenses)

Which two aberrations are controlled with lens form?

curvature of field

oblique astigmatism

What are the two terms of lenses that control for curvature of field and oblique astigmatism??

Corrected-Curve

Best form

Curvature of image is what kind of error?

spherical error

Oblique astigmatism is what kind of error?

astigmatic error

At what viewing distance do we usually try to eliminate oblique astigmatism?

distance

Eliminating oblique astigmatism at near would require?

lens design that is 2.00D flatter

What are the assumptions for lens design?

1. the center of rotation of the eye is a fixed point with respect to the cornea

2. The optic axis of the lens passes through the center of rotation of the eye

3. The foveal chief ray passes through the center of rotation of the eye

Are the assumptions made for a point focal lens design true?

not 100% true

What are the advantages of using a minus-cyl lens design?

less meridional magnification

better cosmetic appearance

Modern corrected curve lenses use which line on the Tscherning Ellipse??

Ostwalt (flatter curve)

Base curve selection is a compromise between?

use of a single BC for alll lenses and a different BC for each lens power

In theory, what should the BC of a high plus lens be?

very steep

What are the problems with using a very steep BC?

thicker/heavier lens

increased vertex distance

more magnification

cosmesis

What are aspheric lens design used for in high plus lenses?

control oblique astigmatism in lens powers that fall beyond Tschernings ellipse

What are tha advantages of aspheric design?

correct oblique Astigmatism

decreases magnification

thinner lenses

better cosmesis

What are the disadvantages of aspheric design?

higher risk of oblique astigmatism from improper fitting

cannot use prism by decentration

expensive

What lens powers should aspheric lenses be considered for?

>+/-4.00 D