Opt 222B Concept Questions For Mt 1 & 2

Front 1

How does the aperture size or location affect the magnitude of spherical aberration?

Back 1

Location -- NO EFFECT

Size -- decrease size, decrease SA

Front 2

What are 2 general forms of lens aberrations, and how can each be minimized?

Back 2

Monochromatic -- change lens FORM

Chromatic -- change lens MATERIAL

Front 3

With reference to lens aberrations, what are the 2 philosophies and their differences?

Which is more preferred?

Back 3

Point-focal -- decrease CI (Pt = Ps, MOP ≈ Pv). Steeper lens.

Percival -- decrease SA (MOE = 0). Flatter lens, so more preferred.

Front 4

What is power error an indicator of and what 2 reference points is it measured from?

Back 4

Power error = curvature of image.

Dioptric difference between Petzval surface & Far Point Sphere.

Front 5

When used with ophthalmic spectacle lens designs, what does the term BASE CURVE really refer to?

Back 5

Standard front curve ground into lens for a grouping of powers for which the front curve is designed to minimize lens aberrations.

Front 6

When fitting an aspheric lens, list 3 things you should be more careful about?

Back 6

1. Monocular PDs
2. Vertical OC (1 mm drop per 2° tilt)
3. 0 decentration for prism
4. Vertex distance
5. Panto tilt

Front 7

In selecting the optimum front curve for a toric lens, what factors of the lens Rx might a designer choose in order to create a lens form that minimizes aberrations?

Back 7

1. Spherical component meridian
2. Cylindrical component meridian
3. Spherical equivalent of Rx

Front 8

What were Jalie's assumptions for each category to eliminate MA?
a) viewing distance
b) eye's center of rotation
c) index of refraction

Back 8

a) infinity
b) 27 mm
c) 1.523

Front 9

In the "family of Tscherning ellipses," each ellipse is located in a slightly different position. Give 3 specific reasons for the variances in the ellipse positions.

Back 9

1. Fixation distance
2. Lens thickness
3. Angle of view
4. Lens index
5. Vertex distance/stop distance

Front 10

What is the difference between the front surface design of an ASPHERIC (+) and (-) lens?

Back 10

Aspheric (+) -- surface FLATTER towards lens edge

Aspheric (-) -- surface STEEPER towards lens edge

Front 11

How might an atoric lens (Rx -2.00 - 1.00 x 180) be designed so it is technically considered superior to a best form lens for the same Rx?

Back 11

With best form lens, the base curve selected for a toric lens is a compromise curve that best resolves the MA induced by each of the 2 principal curves on the backside.

However, with an atoric lens, the back has ***2 separate aspheric curves where each curve maximally reduces the different magnitudes of MA in each meridian.***

The result is a lens which offers less overall peripheral MA, hence a lens that optically is better and offers a ↑ FOV, along with the benefits of a thinner/flatter lens.

Front 12

Why is spherical aberration not a function of the stop position?

Back 12

SA is a function of stop SIZE, not location, b/c SA involves AXIAL RAYS. Changing the location of the stop would not change the amount of axial rays coming in.

Front 13

What is another name for best form lenses and what meaning does it have in relationship to lenses we prescribed for a patient?

Back 13

Corrective curve.

Most optimal front surface curvature (BC) to eliminate lens aberrations.

Front 14

Under what circumstance(s) might an atoric lens design prove to be of little if any value?

Back 14

If the lens is non-toric/SPHERICAL (no cylindrical power), then atoric lens would not be of any benefit.

Front 15

From a lens designer standpoint, identify 2 circumstances that would make the following statement true:

"The MOE is equal to zero."

(mean oblique error = power error = image shell error = curvature of image error)

Back 15

1. FPS = PS
2. MOP = FPS
3. MOP = Pv

Front 16

If MOE = 0, whose design philosophy does it satisfy?

Back 16

Percival's

MOE = 0, CI = 0

Front 17

Indicate how an aperture stop located at the (a) lens front surface, (b) in front of the lens front surface, and (c) behind the lens front surface influences each of the following aberrations:

1. Spherical Aberrations
2. Distortion
3. Marginal Astigmatism
4. Coma

Back 17

Indicate how an aperture stop located at the (a) lens front surface, (b) in front of the lens front surface, and (c) behind the lens front surface influences each of the following aberrations:

1. Spherical Aberrations
NO EFFECT anywhere

2. Distortion
Lens front surface -- No distortion
In front of lens front surface -- Barrel distortion
Behind lens front surface -- Pincushion distortion

3. Marginal Astigmatism
Lens front surface -- MA present
In front of lens front surface -- INCREASE MA
Behind lens front surface -- DECREASE MA

4. Coma
NO EFFECT if SA is corrected
Effect if SA is not corrected

Front 18

When analyzing a Tscherning ellipse graph, you note a lens has its base curve located inside the CENTER of the Tscherning ellipse instead of on the ellipse. How would you change the lens BC for it to fall on the WOLLASTON BRANCH of the ellipse?

Back 18

Steepen BC

Wollaston is on top (steeper), Ostwalt on bottom (flatter)

Front 19

Where is the position of the Petzval surface relative to the Far Point Sphere? Explain the diff b/t these 2 surfaces.

Back 19

PS is a fxn of the lens.
FPS is a fxn of the eye.
PS is flatter than FPS and represents CI b/t the two surfaces.

Front 20

Which lenses have slaboff line on the FRONT surface?

Back 20

SV glass
Fused glass FT
One-piece glass with seg on back

Front 21

Which lenses have slaboff line on the BACK surface?

Back 21

SV plastic
MF plastic
PALs
Exec. MFs (glass & plastic)

Front 22

Where is the REVERSE slaboff line on plastic lenses? Glass?

Back 22

BOTH plastic and glass have reverse slaboff line on FRONT surface.

Front 23

Given a (+) power sphero-cylindrical Rx lens, identify 3 possible lens design concepts. While considering the Rx, contrast the benefits of each.

Back 23

Aspheric and Atoric????

Front 24

Explain how a lens that has MA can be reduced by using another lens design concept. Identify this lens concept and explain what is done to reduce the MA.

Back 24

Use point focal lens. Pt = Ps, MOP = Pv. Corrects MA completely but leaves power error (CI).

Front 25

Tscherning's Ellipses -- there is a difference in size & location from one plot to the other. What could explain this? (5 things)

Back 25

1. Refractive index (of lens)
2. Thickness (of lens)
3. Stop distance
4. Fixation distance
5. Viewing angle

Front 26

How is the Petzval surface affected as one moves the aperture stop to different locations from the lens?

Back 26

No effect??

Front 27

If aperture increases from 2 to 4 mm, how much increase is the magnitude of spherical aberration (LSA & TSA)?

Back 27

LSA = aperture^2 --> 4 to 16 = 4x
TSA = aperture^3 --> 8 to 64 = 8x

Front 28

Annoying reflections may be resolvable through either steepening the BC in some situations, or flattening the BC in others. What explanation can you give for this claim?

Back 28

Moves reflection off visual axis. Changing BC shifts image to less noticeable area.

Front 29

Why does reducing the aperture size help control spherical aberration?

Back 29

SA is due primarily to peripheral rays. Decreasing aperture size limits peripheral rays.

Front 30

What is the difference between power error and mean power error (also MOE)?

Back 30

Power error = measure of magnitude of CI (difference between FPS & PS)

MPE/MOE = also measure of CI (difference between MOP & Pv)

Front 31

According to the Tscherning ellipse there are 2 possible options for any given lens power. What are these and how do they differ?

Back 31

1. Ostwalt --> flatter BCs
2. Wollaston --> steeper BCs

Front 32

What is mean oblique power?

Back 32

The dioptric midpoint between Pt & Ps, where MA = 0.

Front 33

As (+) lens power increases, how should the BC of lens change to minimize lens aberrations? How about as (-) lens power increases?

Back 33

(+) lens power increases --> steeper BC to minimize aberrations

(-) lens power increases --> flatter BC

Front 34

What type of distortion is expected for a (+) lens when the aperture is placed in FRONT of the lens, and when the aperture is placed BEHIND the lens.

Back 34

(+) lens

Aperture in FRONT of lens --> barrel

Aperture BEHIND lens --> pincushion

Front 35

LCA for the eye is relatively unimportant in comparison to LCA for spectacles. Why?

Back 35

Decrease aperture size (pupil) helps decrease LCA. Pupil is smaller than spectacle aperture stop, so LCA for eye is less significant.

Front 36

Which light rays (paraxial, peripheral, oblique, parallel) are most responsible for each of these aberrations:

1. Spherical aberration
2. Coma
3. Marginal astigmatism
4. Distortion

Back 36

1. SA -- parallel, peripheral
2. Coma -- Oblique, peripheral
3. MA -- oblique, paraxial
4. Distortion -- parallel, peripheral

Front 37

What is the significance/advantage of prescribing an ATORIC lens over an aspheric lens?

Back 37

1. Increase field of view
2. Better aberration reduction
3. Thinner lens
4. Less magnification

Front 38

When fitting a patient with aspheric lenses, why might we say VD concern is a "two edge sword"?

Back 38

A benefit of aspheric lenses is that they offer a larger field of view. But because they are flatter, they lie closer to the patient’s face. In order to correct this, we must increase the vertex distance, but this in turn reduces the field of view. So you are back to where you started.

Front 39

How would you confirm (by lensometry) if a pair of glasses requiring complete vertical imbalance neutralization at the READING LEVEL has been fabricated properly?

Back 39

1. Dot segments at RL
2. Put non-slab-off lens with dot over lens stop
3. Center target with compensating prism
4. Move to slab-off lens with dot over lens stop (don't move stage)
5. If 0 imbalance, target should be in center. If not, read off amount of residual prism.

Front 40

List 4 factors of a lens that may contribute to the presence of monochromatic aberrations.

Back 40

1. Lens [aperture] size
2. Lens VD [aperture stop location]
3. Obliquity of light rays
4. Pantoscopic tilt
5. OC position
6. Lens thickness/index

Front 41

You have 3 separate patients in your practice that had relatively equal REs in each eye last month. However, each of them is now anisometropic due to some surgical procedure. Give 3 different possible procedures.

Back 41

1. Cataract surgery
2. Refractive surgery
3. Scleral buckle

Front 42

Besides using slab off or reserve slab off to compensate for vertical imbalance, what are other methods you could use to minimize prismatic effect?

Back 42

1. Dissimilar segments
2. CLs
3. Lower DOC (to split prism)
4. R-compensated segs
5. Fresnel prisms (in one lens)
6. 2 pairs SV Rx (D/N)

Front 43

Why are SA and coma not significant contributors to the total lens aberrations, while MA and curvature of field are?

Back 43

SA & coma are aperture-dependent. Since eye's aperture is small, it limits peripheral rays to decrease SA & coma.

Front 44

Describe the procedure for determining the MAGNITUDE of slaboff in the lensometer.

Back 44

Straddle the slaboff line over the lens stop (OUTSIDE the segment). The separation between the centers of the 2 targets is the magnitude of slaboff.

This is NOT the same as determining the amount of vertical imbalance at the reading level!

Front 45

How does MA change:

1. When viewing angle decreases?
2. When viewing distance increases?
3. When aperture stop location decreases?
4. When aperture stop size increases?
5. When BC of lens decreases?
6. When lens design is changed from spherical to aspheric?

Back 45

How does MA change:

1. When viewing angle decreases? Decrease.
2. When viewing distance increases? Decrease.
3. When aperture stop location decreases? Decrease.
4. When aperture stop size increases? Increase.
5. When BC of lens decreases? Increase.
6. When lens design is changed from spherical to aspheric? Decrease.

Front 46

Describe the design of a meridional afocal eikonic "size" lens.

Back 46

Afocal lens with a bitoric design (toric on both sides). It corrects magnification differences in only one meridian only and not the other. It completely eliminates the magnification in one meridian and has 0 power in the other meridian. Pv = 0

Front 47

You suspect your patient is an axial myope. If corrected with spectacle lenses, how do you compare the patient's retinal image size relative to the emmetrope's retinal image size?

Back 47

Axial myope w/ spectacles will have the same retinal image size as an emmetrope.

Front 48

If fitting an aphake or high plus patient, what are 3 things in a frame you should consider when selecting a frame?

Back 48

1. Frame PD = Patient PD (small eye size)
2. Adjustable nosepads to change VD and panto tilt
3. Frame as circular as possible

Front 49

Aphakic lens fitting considerations

Back 49

1. Vertex distance
2. Panto tilt
3. OC position
4. Lens weight/thickness (material/size)
5. Frame (nosepad, small eye size, circular)
6. Multifocal round seg better (and set seg higher)

Front 50

Best possible cosmetic appearance for spectacle lenses:
High myope vs. high hyperope

Index of refraction & asphericity, in order of priority consideration.

Back 50

High myope -- INDEX first, then asphericity

High hyperope -- ASPHERICITY first, then index

Front 51

The edge thickness of a full-field (+) lenticular lens is the same as a full-field (-) lenticular lens. Why is this true?

Back 51

The edge thickness of a lenticulated lens is the same in (+) and (−) lenses b/c in a lenticulated lens, there is a central portion (~30 mm) where all the power is. The edge of the lenticulated part does not have to be as thick or thin as a regular full-field lens because it is usually just a plano carrier. You can make the edge thickness the same size.

Front 52

What is the difference between field of fixation in a high hyperope vs. a high myope?

Back 52

Myope --> larger field of fixation
Hyperope --> smaller field of fixation

Front 53

Which segment style is optically more appropriate when fitting an aphake with a bifocal? Why?

Back 53

Round seg → minimizes differential displacement

Major lens -- Base UP
Round seg -- Base DOWN
(or Ultex, or any seg with high r-value)

Helps to negate prism from major lens

Front 54

In the RevOptom article, what surgeries did the patient have, what was the cause for the patient's anisekonia, and what optical lens design was used to minimize it?

Back 54

1. Cataract surgery (IOL implant) --> retinal detachment --> scleral buckling (cause for aniseikonia Sx). IOL exchange for mislocated IOL.

2. Eikonic (bitoric) lenses

Front 55

Is pseudophakia a case of refractive or axial anisometropia?

Back 55

Refractive

Front 56

According to the RevOptom article, describe the specific spectacle Tx modality that helped minimize the patient's complaint.

Back 56

Meriodional Eikonic Lenses
○ Bitoric lens design
○ Increase magnification in a particular meridian
○ Afocal → has no power in either meridian

Front 57

Identify 4 factors that will influence the impact resistance of a lens.

Back 57

1. Lens coatings
2. Lens thickness
3. Lens surface characteristics
4. Lens Rx and sphericity
5. Lens material physical characteristics
6. Lens support
7. Size of object impacting lens
8. Velocity of object impacting lens

Front 58

Describe the dress safety drop ball test.

Back 58

Lens must be able to withstand the impact of a 5/8" steel ball weighing 0.56 oz. dropped from a height of 50" onto the horizontal upper surface of the lens.

Mainly used for glass lenses; CR-39 and hi-index may be batch-tested.

Front 59

When the reverse slaboff is used on a lens, what did you do to the lens optically?

Back 59

Add BD to the most (+) lens. Shifts image upward in that eye.

Front 60

Describe what a bitoric spectacle lens is - what is its purpose and why/when might one use this lens?

Back 60

Bitoric spectacle lens designed with toric front & back
Increases magnification in one principal meridian compared to the other.

Purpose is for patients who experience aniseikonia due to meridional magnification, which can occur after cataract or RD surgery. This lens can be used to equalize the amount of meridional magnification seen between the 2 eyes.

Front 61

What are the 4 most likely reasons for developing aniseikonia, according to the RevOptom article?

Back 61

1. Pseudophakia
2. Scleral buckle
3. Vitrectomy
4. Asymmetrical convergence

Front 62

When you have an axial ametropia situation and you wish to provide glasses to minimize the magnification differences, what would be most practical for you to do?

Back 62

Axial aniseikonia is caused only by the SHAPE of the lens.

Decreasing BC or thickness, and/or increasing index of refraction will help to minimize magnification.

Equalize BC/CT of both lenses

Front 63

Define Knapp's Law.

Back 63

In axial ametropia, if the nodal point (2°) of spectacle lens coincides with the focal point of the eye (anterior fp or 1° fp), then the retinal image height is the same as in a standard emmetropic eye.

Front 64

What are the clinical implications of Knapp's Law?

Back 64

If anisometropia is axial, then expect no (or very little) spectacle magnification from the power components. Also, magnification differences should only come from the shape component. Do not expect aniseikonia problems with spectacle correction for axial ametropes unless due to shape differences.

Front 65

For a high myope, describe how wearing glasses rather than CLs would affect your clinical measurements of:
a) fusional reserves (PRC, NRC)
b) amplitude of accommodation
c) visual fields
d) visual acuity

Back 65

For a high myope, describe how wearing glasses rather than CLs would affect your clinical measurements of:
a) fusional reserves (PRC, NRC) -- INCREASE (wider)
b) amplitude of accommodation -- INCREASE (greater)
c) visual fields -- unable to stimulate peripheral retina; very peripheral field targets will be imaged on mid-periphery of retina; blind spot larger and displaced outward
d) visual acuity -- DECREASE

Front 66

List optical consequences resulting from magnification from strong (+) lenses to correct aphakia.

Back 66

1. Improves VA
2. Causes image motion
3. Reduces field of view (or creates ring scotoma)
4. Increases convergence demand
5. Reduces accommodation amplitudes (↑ accommodative demand)
6. Gives pincushion distortion
7. Makes eyes look big
8. Enlarges entrance pupil (brighter image)

Front 67

What is the difference between an ATORIC lens surface and a TORIC lens surface? What is the significance or benefit of an atoric surface?

Back 67

Each meridian of the toric surface is made aspheric. Each aspheric meridian's goal is to minimize lens aberrations, specifically MA & CI. By reducing as much aberrations as possible in each principle meridian, effectively the overall field of vision is perceived by the patient as being clearer and better.

Front 68

List general optical concerns related to high power (+) lenses not as prevalent in lower power lenses.

Back 68

1. Greater accommodative demand
2. Greater convergence
3. Greater object movement
4. Decrease FOV/FOF
5. Increase aberrations
6. Ring scotoma potential for powers > +12 --> ring scotoma; creates appearance of constricted field in VF testing; blind spot smaller and displaced in
7. Pincushion distortion more apparent
8. Magnification
9. Greater lens thickness (cosmetic/weight)

Front 69

List symptoms most commonly associated with aniseikonia

Back 69

1. Asthenopia
2. HA
3. Photophobia
4. Reading difficulty
5. Nausea
6. Diplopia
7. Nervousness
8. Decreased depth perception

Front 70

What are 2 psychophysical tests that can be used to measure aniseikonia?

Back 70

1. Stereo distortion tests (Eikonometer)
2. Size difference tests (anaglyphs)

Front 71

Would you expect a 15D myope to have a brighter or duller retinal image when they use spectacles compared to CLs?

Back 71

Brighter b/c entrance pupil of eye/spec system is larger than entrance pupil of eye alone.

Front 72

What is the max add power available today in PALs? FT? RT?

Back 72

PALs -- +4.00 D
FT BFs -- +6.00 D
RT BFs -- +20.00 (+30.00)

Front 73

What is a quick way to identify whether a lens has been heat tempered?

Back 73

Place lens under a polariscope (colmascope). Look for "Maltese" cross pattern.

Front 74

When designing an aspheric lens, how would you describe the optical and physical difference between a “mildly” aspheric lens and a “severely” aspheric lens in the same power? How might the difference in these two designs impact the fitting of the lens on a patient?

Back 74

“Mildly” aspheric: front base curve would not be as flat as the more “severely” aspheric; would have a larger “sweet spot” (clear optical zone) than the more aspheric lens; may not theoretically correct higher order aberrations as well; may not correct aberrations along each principal meridian like an atoric lens would; this lens is physically steeper and thicker than the more aspheric lens.

Fitting issues: the more aspheric lens is flatter and thus likely to sit closer to the eye which may cause eyelashes to touch the concave surface of the lens; must be more precise in the fitting of the more aspheric lens due to the smaller “sweet spot”; may need to adjust the vertical placement of the lens optical center so that it is not too far off the pupil center; prefer to use monocular PD’s, more critical in the highly aspheric lens.

Front 75

The mechanism by which glass fractures may be different depending on size and velocity of the object. What are the two mechanisms that best explain lens fracture in a minus and plus lens?

Back 75

Rear surface fracture: moderate mass, moderate velocity object; due to lens flexure; stress transferred from point of impact to back surface; most likely fracture mechanism on DBT with MINUS lenses

Edge fracture: large mass, low velocity objects; lens flexure causing lens to flatten; fracture at lens edge; PLUS lenses (not wave propagation)

Front 76

What is the current ANSI standard requirement with reference to the thickness of a glass lens that meets the impact resistance test?

Back 76

There is NO thickness requirement.

Front 77

How does standard impact testing differ from static testing?

Back 77

impact testing: a given energy load is applied to a single lens and additional energy load is repetitively increased until lens breaks (or a new lens is used for each additional level of energy load)

static testing: energy load is applied continuously to lens until fracture/breakage takes place