3 - Aberrations

Question 1

Describe monochromatic aberrations

Answer 1

Wavelength-independent

Either:

• distort image quality (spherical, coma, radial astig)
• deform image plane (curvature of field, distortion)

Question 2

Spherical aberrations

-basis for SAs

Answer 2

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

Question 3

Describe marginal rays

Answer 3

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

Question 4

Describe longitudinal spherial aberration

Answer 4

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

Question 5

Describe coma

Answer 5

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

Question 6

Spherical aberration and coma

• when they’re ignored
• when they’re considered

Answer 6

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

Question 7

General uses for aspheric lenses (4)

Answer 7

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

Flatten lens - cosmetically appealing bc reduces magnification

Reduce weight of lens

Progressives

Question 8

Describe radial astigmatism

Answer 8

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

Question 9

Describe Tscherning ellipse

Answer 9

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)

Question 10

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

Answer 10

Tangiential
Sagittal
Petzval

Question 11

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

Answer 11

Curvature of field

Radial astigmatism

Question 12

Describe curvature of field

Answer 12

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

Question 13

Minimizing curvature of field (2)

Answer 13

Good base curve (Ostwalt)

Use curved screen - e.g. retina

Question 14

Describe Petzval surface

Answer 14

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

Question 15

Point focal lens vs percival form lens

Answer 15

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

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

Question 16

Describe distortion

Answer 16

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)

Question 17

Distortion

• minus lens
• plus lens

Answer 17

Minus = barrel

Plus = pincushion

Question 18

Minimizing distortion

Answer 18

Orthoscopic doublet

Question 19

Chromatic aberrations

• result from
• visible light long -> short wavelengths
• which bends more

Answer 19

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

Question 20

Chromatic aberrations

-who will notice

Answer 20

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

Question 21

Underlying concept used in red-green balance in clinical refraction

Answer 21

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

Question 22

Abbe number

• what is it
• equation

Answer 22

Quantification of chromatic aberration

CA = F/υ

Chromatic aberr = power of lens ÷ abbe number

Question 23

Achromatic doublet

-describe

Answer 23

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

Question 24

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

Answer 24

Shorter vertex distance
Monocular PDs
Sufficient panto tilt

Question 25

Aberrations of most concern (3)

Answer 25

1) oblique/radial astig
2) curvature of field
3) distortion