Flashcards in 3 - Aberrations Deck (25)

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1

##
Describe monochromatic aberrations

###
Wavelength-independent

Either:

-distort image quality (spherical, coma, radial astig)

-deform image plane (curvature of field, distortion)

2

##
Spherical aberrations

-basis for SAs

###
Geometric optics is based on PARAXIAL APPROXIMATIONS (near middle)

-incident rays are close to the optical axis, yielding point images for point objects

Reality = paraximal approximations are not always valid

3

## Describe marginal rays

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Rays on the periphery (not close to optical axis)

Bent MORE than central rays -> focus closer to the lens compared to central rays

***BASIS OF SPHERICAL ABERRATIONS***

4

##
Describe longitudinal spherial aberration

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Marginal rays focus to a different location compared to paraxial rays

-essentially a point object is no longer forming a point image

-results in IMAGE BLUR

-occurs for both on- and off-axis points

-contributes to NOCTURNAL MYOPIA

5

## Describe coma

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Occurs ONLY FOR OFF-AXIS POINT SOURCES

Results from the fact that MAGNIFICATION IS VARIED as the height of incident rays above the axis is varied

-result is an asymmetric comet-shaped patch

6

##
Spherical aberration and coma

-when they’re ignored

-when they’re considered

###
Ignored: designing ophthalmic lenses

-bc the small pupil size only accepts paraxial rays

Considered: very high-powered lenses (+10 or more)

-necessary to compensate for spherical aberration by using aspheric lenses - modify the lens surfaces w/o changing power

7

## General uses for aspheric lenses (4)

###
High-powered lenses (less than -23 or greater than +7)

Flatten lens - cosmetically appealing bc reduces magnification

Reduce weight of lens

Progressives

8

## Describe radial astigmatism

###
aka oblique/marginal

Due to RAYS HITTING the lens/interface OBLIQUELY

Power is altered by this “tilt” of the lens

Tangential and sagittal rays are altered asymmetrically

Reduced by picking correct base curves

Associated with teacup and saucer image

9

## Describe Tscherning ellipse

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Collection of points on base curve vs Fv plot which shows the best value of the base curve for eliminating oblique astigmatism

Specifics vary with viewing distance and material

2 curves:

-Wollaston

-Ostwalt (flatter)

10

## 3 types of curved images produced by radial/oblique/meridonal astigmatism

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Tangiential

Sagittal

Petzval

11

## _(2)_ can be minimized by choosing the correct base curve (primarily from the Ostwalt curve)

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Curvature of field

Radial astigmatism

12

##
Describe curvature of field

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aka power error

The image plane is warped even for a lens that’s not tilted -> quality of an image on a flat screen decr for larger distances

E.g. old-school projector: edges of image on screen are blurry

-due to curved image being projected

13

## Minimizing curvature of field (2)

###
Good base curve (Ostwalt)

Use curved screen - e.g. retina

14

## Describe Petzval surface

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Image surface created by a system with no radial astigmatism

Still warped due to curvature of field

For a thin lens in air:

K = F ÷ n

Curvature of image surface = power of lens ÷ index of refraction

15

## Point focal lens vs percival form lens

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Point: lens corrected completely for RADIAL ASTIG (curv of field uncorrected)

Percival: lens corrected completely for CURVATURE OF FIELD (radial astig uncorrected)

16

## Describe distortion

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Does NOT cause blur or poor resolution

Results from the fact that magnification of a point object depends on the objects distance from the optical axis

Straight line objects form straight line imagees only if the line passes thru the optical axis

All other lines are curved

Problem for high powered lenses (e.g. aphakic pts)

17

##
Distortion

-minus lens

-plus lens

###
Minus = barrel

Plus = pincushion

18

## Minimizing distortion

### Orthoscopic doublet

19

##
Chromatic aberrations

-result from

-visible light long -> short wavelengths

-which bends more

###
Refractive index (n) is slightly dependent on wavelength

ROYGBIV = long to short wavelength

SHORTER wavelengths (blue, violet) bend MORE as they pass thru an interface/optical system than longer

20

##
Chromatic aberrations

-who will notice

### Pts with high-powered lenses may see colored fringes around objects

21

## Underlying concept used in red-green balance in clinical refraction

### Chromatic aberration - green focuses in front of red in the eye

22

##
Abbe number

-what is it

-equation

###
Quantification of chromatic aberration

CA = F/υ

Chromatic aberr = power of lens ÷ abbe number

23

##
Achromatic doublet

-describe

### Combo of 2 lenses: one (+), (-); each with a diff material such that chromatic aberrations cancel each other out

24

## When considering lenses with low Abbe number, what conditions may be helpful in minimizing effects of chromatic aberration (3)

###
Shorter vertex distance

Monocular PDs

Sufficient panto tilt

25