Spectacle Lens Aberrations

longitudinal chromatic aberration

dioptric difference b/t blue light & red light (image spread along the optical axis)

longitudinal, lateral

what are the chromatic aberrations?

lateral chromatic aberration

difference in size b/t images formed by 2 different wavelengths, such as red & blue light

spherical aberration, coma, oblique astigmatism, curvature of field, distortion

what are the monochromatic aberrations?

oblique astigmatism, curvature of field, distortion

which monochromatic aberrations are problematic for spectacles?

nearly the same

materials w/ low chromatic aberration have ______ index of refraction for each wavelength

abbe value

most commonly used # for identifying the amount of chromatic aberration for a given lens material

higher

the ______ the abbe value, the less chromatic aberration in the lens

1. crown glass/CR-39 plastic

2. trivex

3. polycarbonate

rank the following materials from highest to lowest Abbe value:

polycarbonate, CR-39, trivex, crown glass

crown glass

what material has the highest Abbe value?

polycarbonate

what material has the lowest Abbe value?

=

chromatic power _____ lateral chromatic aberration when referring to prismatic effects

increase

as prismatic effects increase, the effects of chromatic aberration ______

increased, more

w/ increased prismatic effect, comes ________(increased/decreased) chromatic power & ______(more/less) image blur

lower

higher chromatic aberration, ______ abbe value

faster

the lower the abbe value, the ______ reduction in relative VA peripherally

1. use monocular interpupillary distances

2. measure MRP heights considering pantoscopic angle

3. use shorter vertex distances

4. have sufficient pantoscopic angle, but not more than 10deg for high lens powers

5. give attention to comparative edge thicknesses

what are the important dispensing factors for lenses w/ low abbe values?

low

polycarbonate has a ____ abbe value

achromatic lenses

plus lens of one material & minus lens of another material with different refractive indices that counteract the effects of chromatic aberration, lens powers must add up to needed power

no, standard spectacles have really good central optics & CA are only an issue peripherally so we don’t really care about them there

are achromatic lenses used for ordinary spectacle lenses? why/why not?

spherical aberrations, coma, oblique astigmatism, curvature of field, distortion

what are the Seidel aberrations?

spherical aberration

Seidel aberration type: peripheral rays focus at different points on the optic axis than do paraxial rays

pupil size limits the number of rays entering the eye for any given direction of gaze

why is spherical aberration not a problem in ophthalmic lenses?

coma

Seidel aberration type: light from peripheral areas of the lens is focused farther from the true image point than it should be, light farther in the periphery is displaced increasingly farther from the point focus; so there is a single point image but it appears comet shaped

pupil size limits the rays entering the eye

why has coma generally been ignored for ophthalmic lenses?

oblique astigmatism

Seidel aberration type: rays from an off-axis point strike a spherical surface at an angle; causes light to focus as 2 line images instead of a single point

finding optimum base curve

how can oblique astigmatism be reduced?

same, higher

when a spherical lens is tilted, the new sphere power will be the ____ sign but _____ in power

same

when a spherical lens is tilted, the new cyl power will be the ____ sign as the original sphere

minus

when a minus spherical lens is tilted, the new cyl power will be a _______ (minus/plus) cylinder

plus

when a plus spherical lens is tilted, the new cyl power will be a _______ (minus/plus) cylinder

same

when a spherical lens is tilted, the new cyl will have an axis that is the _____ as that of the meridian of tilt

180

pantoscopically tilted lenses have a new cyl axis at ____

90

face-form tilted lenses have a new cyl axis at ____

Petzval surface

curved surface that is created when oblique astigmatism is corrected & the tangential & sagittal foci collapse down

curvature of field (power error)

Seidel aberration type: causes lens to have the effect of being off-power in the periphery when worn

image shell error

the dioptric power difference b/t the far point sphere & the Petzval surface; measure of curvature of field

yes

if a designer makes a lens that is completely free of oblique astigmatism, will there still be power error that occurs when looking through the periphery of the lens?

plus

pincushion distortion usually occurs with what type of lens?

minus

barrel distortion usually occurs with what type of lens?

distortion

Seidel aberration type: occurs because there is a different magnification in different peripheral areas of the lens; magnitude is in proportion to the distance of those areas from the optical center of the lens

more

the farther from the center, the _____ distortion is manifested

high, low

chromatic aberration is important for _____ index materials with ____ abbe values

oblique astigmatism, power error, distortion

what are the 3 most troublesome Seidel aberrations?

vertex distance, lens thickness, refractive index, front & back curve powers

what are the variables when designing lenses?

point focal lens

corrected curve lens design: correct the oblique astigmatism completely & leave power error uncorrected

percival form lens

corrected curve lens design: eliminate power error & leave residual astigmatism uncorrected

minimum tangential error form lens

corrected curve lens design: design a compromise b/t point focal or percival form lens

Tscherning ellipse

graphically shows how base curve choices are limited; for a given lens power & distance, there is a BC that will get rid of oblique astigmatism best

Ostwalt

most every traditional spherically based lens is designed w/ a base curve similar to that indicated by the ______ lens form on the Tscherning’s Ellipse

-6.00D

plano lens powers usually have back surface curves close to _____

compromise b/t near & distance working distance

most lenses are designed to have a base curve suited to what working distance?

periphery

differences in image sharpness for different base curves will appear primarily in the _______ of the lens

working distance

the size of the Tscherning Ellipse changes with ________ & thus the BC changes as well

steepens, flattens

as minus lens power increases, the back surface _________ (flattens/steepens) & the front surface _______(flattens/steepens)

flattens, steepens

as plus power increases, the back surface _______ (flattens/steepens) while the front curve ________ (flattens/steepens)

SE + 6.00

Vogel’s formular for plus lenses: base curve =

(SE/2) + 6.00

Vogel’s formula for minus lenses: base curve =

keep as calculated

if the right & left base curves are greater than 2.00D apart, what should you choose as the base curve?

use the base curve for the stronger lens for both lenses

if the base curves are less than 2.00D apart, what should you choose as the base curve?

higher

when the base curves are less than 2.00D for a pair of plus lenses, use the base curve of the _______ plus for both lenses

higher

when the base curves are less than 2.00D for a pair of minus lenses, use the base curve of the _______ minus for both lenses

1. if the frames are large metal or rimless frames

2. if the frame doesn’t hold the lenses securely

3. if the lens has Rx prism greater than 4pd

consider flattening both base curves by 2.00D in what situations?

increase

wrap around lenses ______ the FOV

more, more

for flat lenses, light travels through the periphery of the lens at _____ (more/less) of an angle, resulting in ______ (more/less) chromatic aberrations

less, less

for steep lenses, light travels through the periphery of the lens at _____ (more/less) of an angle, resulting in ______ (more/less) chromatic aberrations

increase, decrease, decrease

wrap frames _______ FOV, _______ chromatic aberrations, & ______ distortion

vertex distance is decreased which decreases the distortion

why is distortion reduced with a wrap frame?

light travels through the periphery at less than an angle which reduces the chromatic aberrations

why do wrap lenses reduce chromatic aberrations?

coma

which of the following Seidel aberration is this:

instead of forming a single point image off the optic axis, the image appears comet or ice cream cone shaped

second

according to the orders w/in Zernicke classification _______ order aberrations are errors which are corrected by written ophthalmic eyeglass prescription

chromatic aberration

causes an image to have a colored fringe

longitudinal chromatic aberration

light of different wavelengths will focus at different focal distances from the lens

lateral chromatic aberration

occurs when a prism bends light 2 different wavelengths by different amounts, resulting in the production of images of slightly different sizes at the focal length of the lens

monochromatic aberration

occurs when the light source contains only 1 wavelength & include Seidel aberrations

T

T/F: the abbe value is the most commonly used # for identifying the amount of chromatic aberration for a given lens material

less

the higher the Abbe value the ______ (more/less) chromatic aberrations

T

T/F: measuring MRP heights & considering pantoscopic angle can help reduce the possibility of chromatic aberration becoming troublesome when dispensing lenses w/ low Abbe values such as polycarbonate

T

T/F: to create a lens that is considered achromatic, a plus lens of 1 material is used in combination w/ a minus lens of another material

spherical aberration

peripheral rays focus at different points on the optic axis than do paraxial rays

coma

causes light from peripheral areas of the lens to be focused farther from the true image point than it should be; light farther in the periphery is displaced increasingly farther from the point focus

oblique astigmatism

when a small bundle of light strikes the lens surface at an angle, the light focuses as 2 line images

curvature of field/power error

light entering the peripheral areas of the lens focuses on the petzval surface

distortion

caused by the difference in magnification in the lens periphery in proportion to the distance to the OC

T

T/F: Zernicke classification of aberrations is used to describe & measure ocular monochromatic aberrations

90

in wrap around frames, the angle of tilt is in the ______ meridian

plus, increase

________ lenses cause pincushion distortion in which the magnification _______ proportionately toward the periphery

T

T/F: spherical aberration is not a huge problem in ophthalmic lenses because the pupil of the eye limits the # of rays entering the eye for a given direction of gaze

highest

when considering right & left lenses as a pair: when one lens is plus & one is minus, from an optical standpoint, the base curve for the lens with the _________ numerical value should be chosen

point focal lens

corrects for oblique astigmatism completely leaving power error uncorrected

percival form lens

eliminates all of the power error but leaves the residual astigmatism uncorrected

minimum tangential error form

corrects for some of the oblique astigmatism & part of the power error

F

T/F: failure to select the correct base curve for a given lens power will reduce the quality of vision only through the center of the lens