Theory content- Lec 1 To Aspherics

Question 1

what is light

Answer 1

part of em sepctrum. stimulates the retina. made up of rays and waefronts. rays represent direction of travel and wavefronts represent the postion the waves have travelled. wave particle duality.

Question 2

what is refraction and why does light bend what about in curved surfaces

Answer 2

light travels in straight lines in all directions and when it hits medium with diff in refractvie index it changes direction and light slows down as it passes from 1 medium to another. Low to high n or deen it bends towards the normal.

Light bends as photons at the end or in the middle slow down less as the front always touches first and changes direction so may speed up or slow down depending on medium it enters. Air-glass it slows down and glass to air it speeds up.

In curved surfaces the middle touches first and slows down so the curved wave front converges.

Question 3

Describe positive and negative lenses

Answer 3

Positive lenses are thicker in the middle than at the edge NIDPIC. These magnify and hyperopes focal point is behind the eye and parallel light focuses behind the retina so positive lenses are needed to converge then light and bring them onto the far point of the eye then focussed by the relaxed eye onto the macula.

Neg lenses thicker at the edge. Generally thin. Myopes parallel light is focused in front of the retina in its relaxed state and parallel light from distant objects is diverged by the negative lens and focussed onto the retina. S

Question 4

What its the second principle focal point

What is vergence

Answer 4

Second principle focal point is the focus point of the far point of the eye. This is where light must be diverging from.

Vergence is the path of curvature of light rays, the more curved it is the greater the vergence. We always get diverging vergence from real objects so negative vergence as light goes from left to right. Object in front of lens.

Question 5

What is BVP
What is ocular vergence vs ocular refraction

Answer 5

BVP of a lens is given by L2’ when the incident vergence so L1 is 0 so distant objects at infinity. The emerging emergence for a near object is not the bvp.

Ocular vergence is the vergence at the corneal apex
Ocular refraction is the vergence at the corneal apex if the initial vergence is 0

Question 6

Define the back vertex power in terms of vergence and why do we use bvp and not principle power

Answer 6

Measure of the vergence of wavefronts of light leaving the back surface of the lens when the vergence of the incident light is 0 so the object is at optical infinity.

We use bvp not principle power as we can measure it on a focimeter and can also measure bvd to corneal apex as its an actual point.

Question 7

Explain in terms of wavefronts how a bi convex lens foccuses light from a distant object and what do we call the value given by the vergence of light emerging from the second surface

Answer 7

Light from distant object means rays are parallel and wavefronts are perpendicular to your rays. Plane wavefront hits front surface of that lens and with a convex lens it means the middle of that plane wavefront will hit the lens first so the light in middle will slow down first and light on edge of wavefront will carry on in air for a little while and will travel faster in the same time period. Wavefronts tell you how far light has travelled in a particular point in time. Wavefront curves inwards or converges as the edge gets further ahead. This carries on till the whole wavefront has passed through the lens= converging wavefront. So air to lens has a higher refractive index n’>n so slows down. Low to high it slows down.

Then its going from the lens to air and n’>n so its now going to lower refractive index high to low which means now once it crosses that boundary the wavefronts will speed up. The edges hit back surface first now and the edges travel faster than the middle of the wavefront in that same time and they get further ahead so the wavefront curves or converges even more. Rays of light perpendicular to the wavefront will focus at a point. So here first the edge ones are going faster as the middle hits and then the edge hits and edge still going faster so if the edge has more time to move ahead= more curved or converging the wavefront will be!

Since we started with a plane wavefront from a distant object, the vergence of the wavefront emerging from the second surface is the back vertex power of the lens.

Question 8

Define the term ocular refraction, b- why is this not the same as the back vertex power of the corrective spectacle lens worn-

Answer 8

vergence at the corneal apex when we start off with an initial vergence of 0 so object at optical infinity.

The back vertex power of a lens is the vergence of light leaving the back surface of the lens when the initial vergence is 0. The corrective lens sits at its back vertex distance away from the eye, the vergence of light as it travels across this back vertex distance changes as we move closer to or further from the image formed by the lens. So the ocular refraction (K) is not the same as back vertex power. There is a distance between the back vertex and corneal apex and vergence changes as rays travel across that distance.

Question 9

When would you need to calculate ocular refraction

Answer 9

If a px with a rx of 4.00d or more wants to wear contact lenses as the power needed for a cl would be significantly (+/0.25 or more) different to the power you found in the trial frame due to it sitting on your eye so you need to account for that change in bvd.

Divergent light becomes less divergent as we move away from the lens image so for a minus lens the ocular refraction will be less than the back vertex power of the correcting lens in the trial frame and so the negative power needed in a cl will be less than the refractive result. decrease BVD need less negative. Increase bvd need more negative.

Convergent light becomes more convergent as we are moving towards the lens image so the ocular refraction needed will be more than that of the back vertex power of the corrective plus and the positive power of a cl will need to be increased compared to refractive result. Decrease bvd need more positive. Increase bvd need less positive.

Question 10

Why is it only necessary to measure the back vertex distance for Rxs over 4.00D?

Answer 10

For low prescriptions the vergence of light leaving the back surface will be low and the back vertex focal length will be longer than for higher power lenses. Up to powers of 4.00D any change in the back vertex distance is negligible compared with focal length and the change in vergence will be small compared to the 0.25D steps by which we can change any prescription. The vergence at the back vertex and the vergence at the ocular surface will be the same if we round to the nearest 0.25D. 4.00D and more, there will be more of a difference in vergence as you travel over that back vertex distance.

Question 11

Define the far point of the eye

Answer 11

Point light must be diverging from or converging to to allow the relaxed eye to focus the light onto the retina. (If the light incident on the corneal apex is to focus onto the retina when the eye is in its relaxed state, no accomodation

Question 12

3) Explain why positive lenses are thicker in the centre than they are at the edge?

4) Where would you find the far point of a myopic eye?

Answer 12

3) Explain why positive lenses are thicker in the centre than they are at the edge?
Positive lenses are thicker in the centre because the front surfaces are more curved than the back surfaces which means the lens surfaces will meet at the edges.

4) Where would you find the far point of a myopic eye?
the far point of a myopic eye is in front of the eye and closer than optical infinity.

Question 13

Describe what prisms do

How can the effect be cancelled out

How can prisms be induced

Answer 13

Prisms bend the light towards the base but they appear as if they are bent towards the apex. So light moves towards the bsae but image moves towards the apex.

Prisms cause the eyes or rotate towards the prism apex. Optical centre is the point on a lens with no prismatic effect.

Horizontal is in and out. HSA VSS vertical up and down. Differential horizontal prism power is power and base but vertical is eye as well.

2)Prismatic effect could be cancelled out if light is still going parallel in the same direction. If different directions then the eyes have to rotate to fix this.

3) prisms can be induced by moving the oc away from the visual axis= induced prism. Or by tilting one lens surface relative to the other to get worked prism.

Prisms can sometimes be wanted proscribed prism control or cosmetic thinning or unwanted due to poorly centred lens or fitted specs. Or NVP in multifocals.

Question 14

combining prism in the sa,e lens and separate lenses

Answer 14

In same lens same base direction so light bends more in that direction and we get additive effect
Opposite base directions light bends in opp directions so effect decreases es or cancels out depending on amount of pris,

Vertical again same thing- same base direc light will bend towards the base more but look as if its from apex.

In separate lenses- differential prism HSA VSS

Question 15

To move ocs out by 1.5cm for each lens is this practical? What other way could we get to required prism

Answer 15

No as this would require a larger blank size and add to the overall thickness of the lens. The lens would be especially thick at the temporal edge, it will be much closer to the optical centre and positive lenses get thicker towards the oc. It would be better to use a semi finished lens and work prism onto the back surface along with the final rx.

Question 16

What is presbyopia explain accomodation,

Emmetrope eye distant object describe the lens and for a near object and presbyopes

Answer 16

Presbyopia- makes the human eye an increasingly focus system and amp of accom decreases with advancing age. Fixed focus so accom reduced, even with no accomodation the eye still has some depth of focus due to small apertures of the eyes which allows us to focus a little closer than infinity.

Emmetropes or corrected ametropia eye for distant objects- ciliary lens is relaxed
Near object- lens fatter and rounder and converges onto retina
Presbyopes- lens cannot change shape cant converge or accom so cant refocus that image onto the retina so need extra add or positive power.

Problem is if too much positive power then focuses in front of retina= pseudo myope then blurry. Ignore cyls as add is a spherical correction

Question 17

Different segment types and development of bifs

Answer 17

Segments are always positive

Down curve/round
D-seg (d28 most common)
E style
C segment- curved top

First distance and near lens chopped in half and stuck together as split bifocal
Cement bifs add portion stuck onto distance
Fused bifs- made of glass smooth fusion between distance and near
Solid bifocals- glass or plastic segment worked onto lens, only round and difficult to manfuacture
Seamless bifocals- edge of lens blended into lens but aberrations
E-seg- large reading area

We want good optics and wearer is looking through oc of d and n= split bifocal. All other lens designs compromise split we can use best form lenses for both but lenses can fall out. Cosmetically seamless is nice and round fused then solid round etc but optically split and e style are the best when they look the worst. Round and seamless worst.

Question 18

Where is the nvp always

Answer 18

Always down a down and in as thats where the segment is. Segment displaced inwards slightly as we converge so we wan to look directly above the segment centre. Inset is a standard 2-2.5mm but not everyone converges by the age amount.
Can alter inset by moving oc in distance portion

Question 19

NVP

Answer 19

Point where the eye converges and drops down to read

ON= near optical centre where prism often cancels out. Sometimes it doesnt exist and if its further from the NVP then we are looking more off axis and we get more aberrations. NVP and ON do not coincide in most multifocals and its not always a real point. Oc of combined distance and near segment.

Question 20

problems with bifs

Answer 20

Poor control of near optical centration= depends on Rx, segment type which controls the amount of prism at nvp

Sudden change in image position (image jump) particularly in round segs. This can be eliminated in vertical directions by suitable segment choice or working base up prism onto segment but not laterally.

Poor va in segment= potential problem in fused due to wavy interface between lens material bc of induced prism

Monochromatic and chromatic aberrations in segment depends on rx and lens segment cannot control independently. Colour fringes at BVP when going for high index lenses generally.

Question 21

Base direction in a d segs- on pg 12 of my notes. What would the base direction be if nvp was em below and 7mm below.

Answer 21

Radius of bottom half of segment is 19 depth is 24 which is the overall size of the segment. 24-19 means the segment centre is 5mm below the top.

3mm down= would be looking above the segment centre so positive lens so base down
7mm-= looking below segment centre which means a base up

Look at position of the NVP to see base direction

Question 22

Bifocal jump and how to eliminate

Answer 22

Just past segment divide. Sudden introduction of prism at the segment top and the image jumps and eye has to refixate on the image. From distant to near and its hidden till the wearer moves their head. Used to it.

Distance from seg top to seg centre times the add. In round or downcurve this distance is the same as the radius.

Jump is independent of the distance rx, only the add. Can be avoided or eliminated by working the prism onto the segment that is equal and opposite to the jump or by having a smaller radius as the segment centre is closer to the segment top so less jump.

Question 23

Jump for each segment

Answer 23

Jump at dividing line for round segs is always base down. For negative rxs they add to the base down induced by the distance portion at the nvp so there is more jump then d segs and image quality at nvp is poorer.

D28- usually base down unless the radius is equal to the depth in which case there is no jump

E seg- no jump if the OD and ON lie on a straight line as no prismatic effect at segment top so no sudden change in overall prism. (Seg centre can be on segment divide so base up at nvp can be cancelled out by base down due to distance)

Semi-circular seg- no jump as centre of segment at segment to

Question 24

Bifocal problems tca

Answer 24

High index means lower v values means higher tca
Larger degree of prism at nvp= high tca
Average value before tca is noticeable= 0.1 dioptres
Solid bifs. Tca= prismatic effect at NVP/ constringence value

Fused bifs- tca must be calculated for 3 components distance recess curve and the high index segment and then added.

Question 25

Bifocal fitting

Measuring bifocal adds

What does 4-250 mean in plastic c seg

Answer 25

Segment top level with lower limbus and lower if px finds seg top too noticeable. Some say 2mm below for d segs. Metal frames adjust heights, may prefer seg top higher if close working demands or lower if occasional near work.

If segment on rear of lens- measure BVP at distance and near and then add is Fnear-F distance. Measure on back. On surface that the segment is on.

If segment on front of lens measure FVP at distance and near and do the same but measured on the front. But distance power is always specified as bvp.

Front surface is +4.00 on a lens measure and the add is +2.50D. Rear surfaced w required sphere or toroidal curve required to get rx. Prisms may be added at this stage. Front normally done and thickness and everything added to back.

Question 26

1)Why can’t bifocal and progressive lenses be supplied as finished lenses for astigmatic rxs?

Answer 26

Bifocal you have a segment which has to be in a particular position, finished lens means they’re both finished. Astigmatic position means you need your cyl power at an axis which means you would have to rotate the lens to get the cyl in the correct axis, and if you have seg or reading area in the correct position and you move the lens for the cyl it wont be in the right place anymore. Thats why varis and bifocals come as semi-finished the front surface has the bifocal or progression on it and then glazing house surface the back surface with the correct powers at the correct axis to give the final rx. It would also not be practical to store lenses with all cyl powers for all axes.

Question 27

2) Why cant you provide prescribed prism by displacing the lens in a multi focal or pal?

Answer 27

it would displace lens and reading area to different position. The inset for bifocals and varis is set at about 2 or 2.5mm. If the lens is displaced to induce prescribed prism, the segment or progression would not be in the correct position and the wearer would not be looking through the centre of the segment or progression when they converge for near.

Question 28

1)Explain why the near vision point and the near optical centre often do not coincide?

Answer 28

The near vision point is the point through which the wearer looks when reading whereas near optical centre is the point where there is no induced prism when the distance portion and the near point are combined for a bifocal- point where if induced prism caused by distance is cancelled out due to induced prism at the segment which will mean the near optical centre.

Dont often coincide as the near vision point is often not the point where they both cancel each other out. They will only coincide if the induced prism due to the segment is equal and opposite due to the induced prism due to the distance portion at the near vision point. This can only occur for positive rxs and round segments or low negative rxs and d segments unless prism control is used.

Question 29

2) Dr L 46 has noticed she cannot read in her specs anymore. Rx of -1.50/-0.25x90 and L= -1.00/-0.50 x 95. Add= 1.25
she wants something which allows her to see in distance and near at same time she wants bifs as her husband had problems with varis. Occupation= university lecturer and hobbies include reading. Discuss her options including type of segment and advantages and disadvantages of each.

B)Will the px be comfortable using her computer screen with bifocals, other options other than bifocals?

Answer 29

Explain she might get on well with varifocals, persuade her to use varis firstly range of wd etc.
D-seg- most commonly used. D28- they are more noticeable than round segments, for negative rxs the base up due to the segment at the NVP cancels out some of the base down prism induced by the distance portion at the NVP. = less induced prism at the NVP.
Round segments- less noticeable than d segs- generally give more reading area, for negative rxs they add to the base down prism induced by the distance portion at the NVP , more jump than D-segs. more prism=looking further away from oc= more off axis aberrations, image quality at NVP= poorer.
E-style- largest reading area, most noticeable dividing line so cosmetically the worst, segment centre can be on segment divide so base up at the NVP cancels out base down due to distance. As the segment centre is on the segment divide there will be no jump.

B) with a +1.25 add she will still be able to see 80cm away so she should be able to see her screen through the bifocal segment but she may also have enough accomodation to see it through the distance portion depending on how far she works from the screen. She may get asthenopic sxs if she needs to accom for long periods.

if she views the screen through the segment it may be uncomfortable as she may have to tilt her head back. So screen position. You could prescribe her single vision rx for long periods of screen work- but in this case unlikely to be needed and in fact if she removed her specs she will be able to read and probably see screen clearly.

Average sphere in the right= -1.50-0.125= -1.625D and far point= 1/1.624= 0.615= 61.5cm working distance. Left- average sphere is 1.00-0.25= -1.25 far point= 1/1.25= 80cm.

Question 30

3) Plastic 25 segment curve top solid bifocal. Distance power is approx Plano in this form. 4-2.50= +4.00 on lens measure and add is +2.50D. For the above semi finished lens if you assume this is a thin lens what would the back surface power be if the back vertex power is Plano.
2.Clearly this is not a thin lens and your value calculated in Q1 is only an approx, how would you need to adjust the back surface power to account for the lens thickness and why.
3. If the required final rx is -4.00D the minimum centre thickness required is 1.5mm and the refractive index is 1,5 what radius of curvature would need to be surfaced onto the back surface.

Answer 30

Thin lens just add front and back vertex together to get answer. Fv’= F1+F2= 0=+4.00+F2… F2= -4.00D

2. The thickness of the lens contributes some positive power since the convergences of light increases as it passes from the front to back surface. Light hitting the back surfaces will have a vergence of greater than +4.00 which means back surface power would need to be more negative than -4.00D as a result. So its positive which means its converging so it adds more positive power so if the front surface is now more positive the back surface would need to be more negative. Rays would meet behind front surface.
3. F1= +4.00D, t=0.015m n=1.5 and Fv’= -4.00D
   use step along or the back vertex power equation to work out F2 and then use r= (n-n’)/F to work out the radius of curvature.

Question 31

Explain difference between worked prism and induced prism

Answer 31

Worked prism= back surface is tilted relative to the front surface so the optical centre is shifted. Rays perpendicular to the front surface but not the back surface.

Induced prism= whole lens is shifted so no longer looking through oc and the rays are not perpendicular to either surface.

Question 32

Differential prism in anisometropia how to work out prism (differential prism)

Answer 32

Interested in vertical because the distance from OD to NVP is greater in vertical than horizontal meridian as eyes converge smaller distance but look down more and also eyes can cope w horizontal rather then vertical.

So work out prism due to distance portion, forget prism due to the segment as long as it’s the same segment for each lens so same segment top, size addition which is mainly always the same. When working out diff prism just do it based on distance rather than due to the segment. So differential prism is independent of segment as long as the segment is same up both.

Question 33

Conventional bifocals vs prism control bifocals

Answer 33

Conventional- same prism at distance and near- conventional bifs, makes life easier for glazing houses. Can work it on back surface or displace the lens. N

Prism at distance only or near only or different prism at distance and near= prism control bifs

Question 34

Uses of prism controlled bifs and examples of prism controlled bifs

Answer 34

Neutralise prism at NVP= centre controlled bifs (moving oc to nvp)
Induce base in prism due to poorly controlled exophoria or convergence insufficiency
Eliminate differential prism induced at the NVP
No jump bifocals= segment OC on the segment dividing line

Franklin split bif- lens split in half like an e seg. Base in prism at near only due to differential prism
Presto bifocal segment is glued onto hole cut in distance lens. Better cosmetically than franklin split.

Question 35

Inset what can changing it do

Answer 35

Changing the inset can change the prismatic effect

Generally set at 2mm for bifs by lens manufac. can increase it to induce base in at near -horizontally.
Inset-convergence times add which is the extra bit of prism induced due to segment
Also have to take prism due to distance into account

Question 36

Summary ab bifs

Answer 36

Round or downcurve seg= increase bd prism larger eg= greater the prism
More prism more aberrations
Round= greater jump compared to D but less visible than d
E largest reading area and if you put seg centre on dividing line it induces base in prism. more visible no jump if OD and ON lie on a straight line.

Question 37

Control prism. Get rid of differential prism in anisometropic lenses

Answer 37

Using unequal segment sizes- vertical not horizontal

In order to eliminate differential prism= 2.5 base down in the le or 2.5 base up in the re. We would need either 2.5 base up in the le or 2.5 base down in the re to cancel them out. Which side would we put the larger segment-

Larger round segment induces more base down so we can put it in the right eye to induce that 2.5 base down we need to cancel out the 2.5 base up in the re.

Largest segment in the lens with the most plus or least negative power in the vertical meridian. Most positive power.

Question 38

What is the largest segment size readily available and what is r22 generally

Answer 38

45mm.
Generally r22 is the smallest segment size generally

Question 39

Prism control again- bi prism bifocal and how
What can it also be used in

Answer 39

Most commonly used for varis
Base up at near, base up worked over the whole of the rear surface.
Base down worked on the distance curve of the front surface

Gives zero prism at distance and base up at near
Bi prism lens used for the eye with the least plus or the most minus
As more minus gives you more base down so trying to cancel it

How-
Base down is slabbed off to leave base up, now we have base up for distance and near but we only want it for near so an equal amount of base up is removed or slabbed off the distance portion leaving no worked prism there and base up for near.

Can also be used w d seg- bi prism or slab off lenses always have base up at near and can also be used to balance differential prism in progressive power lenses. Base up can help or convergence insufficiency.

Question 40

Prism verification and trifocals

Answer 40

If we place a bi prism lens in a focimeter with the base of it in the middle of the lens rest only covering distance rx we get 2 sharp images one above each other. Measure prism. 2 images of the same power will be seen.

Trifocals- mostly use din usa. Enhances the range of clear vision for older presbyopes with limited accom, manufac similar to bif. Special versions can be useful for occupational purposes or lva. Fitted- top segment= intermediate, measure top of the segment just below the pupil.

Question 41

Mr P’s is R) +6.00/+2.00 X 180 L) +6.0/-2.00 X 180 Add 2.00D
He is wearing R22 downcurve bifocals but he finds it hard to read very long time with them and refers svn glass why

Answer 41

At first glance Rx look same but r rx is written in positive cyl and l in negative cyl. Si vertical power is really +8.00 and +4.00 and horizontal power is +6.00D in the right and left

The difference in vertical power will induce vertical differential prism at the nvp which will cause discomfort when reading. In the svn glasses mr p should be looking through the near optical centres when reading and so there will be no induced prism.

Question 42

What is monochromatic light and imperfect image how do we produce a perfect image

Answer 42

Monochromatic light- make the assumption that light is made of one wavelength and helium d line is used for refractive index= makes calculations easier. Paraxial rays are small and make snells law work so these also make calculations easier.

Imperfect image- the deviation of an image from the perfect image can be considered as:
A deviation of the focus of rays from the perfect focus- the Paraxial focus
or the deviation of the wavefront from the perfect sphere

even in an aberration free system perfect image is not formed, light is diffracted by a finite aperture and a point will spread into a small disc like image= diffraction limited image.

To produce a perfect image lens must form a point image on every point on the object and the points must be arranged in a geometrically similar way varying only in mag.

Question 43

First order third order optics

Answer 43

First order optics sin i=i
Accounts for smaller angles. Uses paraxial approximation so snells law n sin i=n’ sin i’ can be used as angle of incidence= angle of refraction.

For third order optics cant assume sin i= i as rays no longer meet at one point. Accounts for larger angles with a bit more accuracy. Sin i= i-i^3/3.

I3/3 is a term included in a equation where light rays that deviate from paraxial optics results in 5 aberrations known a s third order aberrations or Seidel aberrations:spherical aberrations, coma, oblique astigmatism, curvature of field and distortion

Question 44

Spherical abberations

Answer 44

on axis aberrations, axial Seidel aberration
depends on equal prismatic power across the lens, we want prismatic effect at each point to bend the light at every point by the same amount to make sure all the rays converge ant meet.

So this is when prismatic increase is too rapid and not even so the rays focus progressively closer to the lens.Not generally considered in ophthalmic lens design.

For positive lenses, biconvex lens design minimise SA but this gives high levels of oblique astigmatism so not really used. Oblique is worse. Our eyes have lots of positive SA so dont notice this really. Eyes also have small pupils which limits it.

Question 45

Coma

Answer 45

off axis aberration, when px looks obliquely
This is when marginal rays focus closer than paraxial rays.
Comet like image. Increases wiht distance and angle.
Again small pupils limit coma, not really considered but may be for progressive lenses.

Question 46

Oblique astigmatism

Answer 46

off axis , more noticeable as degrades the image
this is when rays come in at an angle or obliquely, we have tangential and saggital rays which form two foci at right angles to each other and the image is at the disc of least confusion.

Lens designers try to bring them two foci together
Astigmatism lies on curved surfaces= image shells. Once corrected two image shells collapse to a single surface producing curvature of field aberration.

But our image shell is curved as well as curved retina which helps us not to notice it as much.
Reduced by using diff lens curvatures or aspheric lenses/ best form.
Further we move off axis greater amount of astigmatism we get

Question 47

Curvature of field

Answer 47

comparing calculations to ideal, focus at far point sphere
comparing these to an ideal image so we want it to focus at fps
we measure from vertex sphere as distance between eyes centre of rotation and the lens surface changes as the eye rotates so vertex sphere should stay the same meaning image will form on the far point of our eyes.
Image is on a curved petzva; surface

Question 48

Define vertex sphere, far point sphere
And tangential focaln length and sag focal length

Answer 48

Vertex sphere- tangential and sag foci are measured from here
radius of curvature- distance between centre of rotation and back vertex of lens

far point sphere- distance from vertex sphere to far point sphere along straight lines passing through eyes centre of rotation

vertex sphere to tangential focal point= tangential focal length
vertex point to sag focal point= saggital focal length
can work out oblique astigmatism= mean of them two. Error= take it away from the back vertex power.

Question 49

Distortion types

Answer 49

barrel- points displaced closer towards optical axis so looks as if going inwards= negative lenses. Edges squares appear closer together and minified and centre appears magnified. Like a barrel. Magnification decreases off axis.

Pincushion- points displayed away from optical axis so outwards towards edges= positive. Magnification increases off axis.

problems in high powered lenses but not generally considered by lens designers as brain readily adapts, myopes prefer flat form lenses eventhough aberrations are greater- esp if used to them.

Question 50

chromatic aberrations

Answer 50

changes with wavelength
refractive index changes with wavelength.
Higher index have lower constringence- more dispersion bc they cause a greater slowing of speed of wavelength -> more dispersion so more dispersive power= ca
v number/ abbé number/ constringence- measure of spread of light over the visible spectrum, higher means worse.
Higher the dispersive value lower the abbé number of v number (worse) (more dispersion so lower constringence)

Two types of ca-
longitudinal CA- not generally problem bc we have a. Lot in eye
LCA= F/v (units are D) = measure of difference in vergence w wavelengths= axial aberration

transverse chromatic aberration- more of a problem, product of prismatic effect at a particular point on a lens. Work out what prism is and divide by constringence. Problem in high index lenses. Prism power at edge of lenses is high and v value low
tolerance threshold for tca is about 0.1 prism dioptres= causes coloured fringes around high contrast images in periphery, low contrast images are more likely to be seen as a blur. Thin lens= problem but most people compromise.

Question 51

Classification of aberrations

Answer 51

can present them in terms of deviation of image wavefront from perfect sphere if measured along radius of perfect sphere to deformed wavefront and the optical path difference is what they represent on diagrams. Measured in micrometers. So optical path difference is measured along radius of refernence sphere.
represented- as whether that wavefront is in front or behind the peer=fect sphere so it tells you how much that image is deformed. Represents all aberrations in one.

Green= no aberration
yellow to red= wavefront in front of perfect sphere
blue= wavefront is behind the perfect sphere

Question 52

Explain the term first order optics

2) what are the assumptions made when using first order optics calculations

Answer 52

For light rays close to the axis in an optical system so paraxial rays, angles in radians will be small enough to make all but the first term in the Taylor series for the sine of an angle negligible. First order optics refers to the use of only the first order in the series so sin i reduces to i and snells law reduces to n’i’=ni. Sin i= i.

2) the assumptions are that the light rays are in a region close to the optical axis and all angles of incidence and refraction are small (5 degrees or less) so that the approximate snells law n’i’=ni can be applied without significant loss of accuracy.

Question 53

1) Define a perfect image in terms of wavefronts
2) Explain what is meant by a diffraction limited image

Answer 53

To form a perfect image, the refracted wavefronts on each point on the object must be perfectly spherical so they converge to a point image (or all the rays from a point object must converge to a point image) and the points must be arranged in a geometrically similar way to those of the object, varying only in magnification.

2) A diffraction limited image is an image free from aberrations. Each point object is affected only by diffraction and spread into a small disc. Diffraction occurs bc of the limiting aperture of the optical system, the smaller the aperture the more diffraction occurs. The limiting aperture could just be the lens diameter. In the eye the limiting aperture is the pupil. Diffraction effects are usually very small and are masked by the aberrations.

Question 54

5) explain the term third order optics

6) explain why third order optics is not completely accurate

Answer 54

Third order optics uses the first two terms in the Taylor series for the sine of the angle (sin i=i-i^3/3!) in snells law, to trace ray paths through an optical system. More accurate ray tracing values. Third order refers to the third order of i in the second term. consist of 5 aberrations. Used for larger angles than first order.

6) bc it neglects all other terms for sin i in the Taylor series. But it gives reasonable accuracy for angles up to around 20 degrees.

Question 55

7) explain spherical aberration in terms of prisms

8) why do lens designers not consider spherical aberrations

Answer 55

spherical aberrations occurs bc the prismatic power at the edges of the lens is either too strong or too weak to bend light rays to the same point. If increases too much positive longitudinal spherical aberration and if it decreases too much negative long spherical aberration. We want even power.

8) As it’s limited by the eyes pupil and its effect is negligible compared with the spherical aberration of the eye itself.

Question 56

9) explain the effect of barrel distortion on an image

10) why isnt distortion generally a problem for spec wearers and when might it be a problem

Answer 56

9) Barrel distortion causes the periphery of the image to be magnified less or minified more than the centre so points in the image are displaced towards the axis and the centre appears to be protruding outwards. points are getting closer to each other towards the edge of the lens so causing Minified effect at edge of lens. Looks like a barrel.

Not generally a problem fro them as the brain adapts to it and so its effects are not noticed. It may be a problem if a spec wearers with a high rx switches from one lens design to another with very different distortion.

Question 57

11) what is the vertex sphere

12) what is the far point sphere

13) as the eye rotates the distance along the principal ray from the vertex sphere to the fps is constant what does it equal

Answer 57

1) imaginary sphere with its centre at the centre of rotation of the eye. It is used as a reference from which the tangential and saggital focal lengths can be measured along the principal ray at any angle of rotation of the eye. The radius of the vertex sphere is equal to the distance from the back vertex of the lens to the centre of rotation of the eye.

12) the fps is an imaginary sphere w its centre of rotation coincident with the centre of rotation of the eye. It represents the position of the far point of the eye as the eye rotates.

13) the distance along the principal ray from the vertex sphere to the fps is equal to the back vertex focal length of the lens.

Question 58

A myope comes in and says he has vision problem, everything’s fine va, rx, lenses are centred correctly for pd but he insists not right, what is the problem, how could you check and what action could you take?

Answer 58

problem could be due to a change in the form of the lenses compared to his old specs which has caused a change in the level of distortion.

we can check this by measuring the surface curvatures with a lens measure and comparing the new lenses to the old,

explain to him that its a different lens design which has caused some changes and it often takes time for our brains to adapt so he should adapt quickly try them for a couple of weeks warning him not to revert back to his old ones, and to return if no improvement within 2 weeks. If not then the lenses can be changed to match the form of his old spectacles.

some myopes prefer flat form lenses eventhough they may have more distortion and poorer off axis performance.

Question 59

Best form lenses

Answer 59

spherical lens with curvature chosen to minimise defects, form does not matter if you are on axis, only off axis. So it works for zero oblique astigmatism, zero curvature error MOE, or zero tangential power error.
This is the solution for one aberration for a specified working distance, centre of rotation and refractive index so parameters.
Not designed to be best cosmetic and may not be best optical solution. Lens makes no difference to image quality if on axis.

If we get rid of one aberration probably still have others so zero oblique astigmatism prob still have some mOE or astigmatism etc. Only works for a number of parameters in general designers choose best one for distance and close up is more likely to be close to on axis whereas distance may be off axis more so except for free form best form lenses is often for distance only.

Question 60

Flat form vs curved lenses and thin vs thick lenses

Answer 60

Flat form= either one surface is Plano or both surfaces convex or concave

Curved lenses- have one surface convex and one concave

Assume thin lenses but if theres thickness then it adds extra positive power as convergence occurs as you go through the lens so to get the same power you have to take off some extra positive bc of the thickness but for these assume thin lenses.

Question 61

3 lens designs for best form

Answer 61

Point focal lens- zero oblique astigmatism but MOE increases off axis as eye rotates. Tang and sag powers together here. Needs to be more steeply curved. Point focal often positive in periphery but non presbyopes can accomodate but presbyopes cant.

Minimum tangential error- tangential power is the same as BVP, astigmatism increases slightly off axis. Modern designs they like this as if vertex distance increases it behaves as a point focal lens and if decreases behaves as a Percival lens. Not as steep. Least chromatic abb.

Percival lens form- MOE= 0 as sag and tangential powers are equal distances away from the bvp so the mean oblique power is equal to the bvp so the MOE is 0. Small amounts of oblique astigmatism. Flatter. Better for presbyopes.

So by changing curvatures= diff off axis performances.

Question 62

Tschernings what did he do and find

Answer 62

demonstrated range of lens forms free from aberrations, used third order approximations
found quadratic relationships between form of lens and its power
later shown if these relationships were plotted-> ellipses

Found accurate from computer plots. Assumes thin lenses. Plot for specific parameters so this one is for a distance object with specific refractive index and centre of rotation.
plots the back surface powers so with 5.00 for example this can either have a rear surface of -7.50 or 14.00D

ostwalt tends to be the flatter solution= lowest front surface power and more common
wollaston- stepper- higher front surface power

if any of those defined parameter change then this changes the solutions so different surface curvatures would need to be chosen. Solutions for near object will not be the same as distant objects as you usually need a flatter form for near.
For multi and varifocals- best form lens is chosen for distance rather than near, SVD can choose the best form for a working distance but compromise for multi and varis unless free form is used
If BVD changes then off axis performance changes. Thats why BVD is important

there are no solutions for powers greater than 7.00D as you cant get good off axis performance with two spherical surfaces. For this px is better with aspheric surfaces. -22 to +7.00D

Question 63

Dispensing considerations myopes form and distortion

Answer 63

Best form isnt always the best solution. Curved for myopes doesnt work too well.

Curved form so one convex and one concave power, so point focal- induces less OAE but get positive MOE so when they look more off axis the power decreases as the moe becomes positive so the lens will be undercorrected in the oblique gaze for myopes so presbyopes wont be able to accomodate.

For the flat form one Plano one negative surface, higher OAE especially 40 degrees out but when they look more off axis the power gets more negative so MOE decreases and the eye is over corrected in an oblique gaze. Non presbyopes can accom to sharpen the image by bringing one of the focal lines on the retina so that’s why myopes can struggle if their best form is changed. Myopes generally prefer flat form.

But distortion also increases at a faster rate for flat forms so for an even higher myope the best form lenses become flatter so difference between changing from flat to steep could be great, px used to aberrations hard to adapt. Here change in distortion would cause problems rather than moe

Question 64

Changing refractive index

What is best to dispense

Answer 64

changing n requires diff curvature designs= possible adaptation problems
higher n= lower constringence= higher tca
never dispense high refractive index unless significantly thinner= depends on eye size of frame as well as power of the lens. Glazing houses can often advise on this and even provide a profile plot of the cut lens for the chosen frame.
Refractive index generally makes more of a diff for negative rxs compared to +ve

Best to sue best form curved as better off axis performance. Distortion will be about the same for any given power
Lens designers consider OAE and curvature of field. Best form lenses combination of f1 and f2 to get rid of aberrations

Question 65

1)Mr B has a Rx of R:+6.50D and L: +7.00DS. He wants to keep the cost of his spectacles to an absolute minimum. How can you keep the thickness to a minimum without adding to the cost beyond that of standard lenses? What compromises might you need to make-

Answer 65

talking about best form, not aspheric or high index. Sticking w standard index lens

think about the lens but also the frame
consider the size and shape of the frame. Plastic or metal. Good choice of frames can have a significant difference to the lens thickness so go over these i think from ol1
does the CD=PD which is important as we wont have to decentre the lens so gives minimum decentration whilst ensuring px is looking through oc, and also gives minimum blank size= reduces the centre thickness
smaller eye size= smaller blank size to start with as well.
frame shape to suit rx, if astigmatism what is the axis so whats the best frame shape
plastic frame- smaller minimum edge thickness which can impact on blank size and centre thickness as well
more on dispensing scenarios for frame choices

as well as frame also need to consider the lenses- optical and cosmetic compromises
high plus rx tend to think of aspheric and high index but px no money

ideally aspheric lenses which would reduce centre thickness while at the same time giving good off axis image quality. But cost more than standard lenses
finished- stock lenses are generally cheaper than surfaced lenses but are supplied in a limited number of blank sizes. If you have to stick to stock lenses choose blank size carefully to keep centre thickness down.
semi finished lenses may be better as back surface can be finished to give minimum edge thickness, usually a small charge.
Could get away w stock lenses with this as its spherical Rx without cyl dont have to worry about rotation to the right axis

best form- high plus requires steep surface curvatures= thick lenses. Could compromise by using flatter form lenses but this will have an impact on the off axis image quality. Discuss this with px explaining it could make the lenses thinner but things will not be as sharp when looking away from the centre of the lenses. Explain this to them.
Also things like lenticulars but not talking about that rn.

Question 66

2) What are the 3 main design philosophies for best form lenses

Answer 66

point focal- zero oblique astigmatism so reduces this as the eye rotates away from the optic axis but MOE increases with rotation

minimum tangential error- aims to have a tangential error of 0 as the eye rotates but will still have some OAE and MOE. This is when the tangential focus is the same as BVP. Tangential vertex sphere power is the same as BVP. Modern designs tend to be of this design because as vertex distance increases it acts as a point focal lens and if decreases it acts as a Percival lens. If BVD changes you still have a good lens. But do get abberational astigmatism which increases slightly with rotation of the eye away from the optic axis.

Percival lens form= MOE of 0, so the MOE is equal to the BVP. But cant get rid of all the astigmatism so small amounts of OAE remain.

Question 67

What are aspherics and what are toroidal lenses

Answer 67

non-spherical single vision lenses which are rotationally symmetrical around their optical axis. Aspheric lens normally has one aspheric surface and one spherical or toric surface depending on the rx.

Toroidal lenses are aspheric but not rotational symmetrical.

Question 68

Why use aspheric lenses

Answer 68

with tschernings ellipse we saw not good off axis performance, showed that OAE could not be eliminated using spherical surfaces with back vertex powers over +7.00D. More recently used for flatter form lenses to be dispensed with acceptable optical properties for low rxs. Flatter form lenses are better cosmetically. so rather than steeper best form use flatter form lenses and use aspheric to correct for OAE. From about 3.00D it helps them look cosmetically better whilst good off axis performance.

Question 69

How do aspheric surfaces work

Answer 69

Aspheric surfaces are astigmatic away from the axis of symmetry so away from optic axis. The actual surface is astigmatic+ corrects for oblique astigmatism caused by oblique gaze through the lens. Surface astigmatism counteracts OAE.

Corrects one aberration at a time not for all of them.

Question 70

+12.00 BVP with spherical lenses what will we see and how can we prevent this

Answer 70

Not going to get best form lens, as eye rotates more tangential the sagittal decreases a bit so we want to correct for tangential, here it becomes more positive so positive MOE so cant accomodate.

To offset the oblique astigmatism we need a lens surface for which the tangential surface power decreases at a greater rate than the saggital power. We would need a surface for which the tangential radius of curvature increases at a greater rate than the saggital radius of curvature. We want flattening. Surface flattening in periphery. Eg many concoid surfaces or convex prolate ellipse

Question 71

Once oblique astigmatism is corrected

Answer 71

No oblique astigmatism but the lens still has some MOE which reduces as you get to the periphery. MOE negative now whereas for spherical lens it was positive so moves it further behind retina so the wearer can accomodate and get sharp image through the edge of the lens. Not as good for presbyopes.

Question 72

Different conic sections and what does the p value tell us

Answer 72

Parabola- parallel to one edge of the cone. Only one
Circle- slice perpendicular to main axis through cone, one
Ellipse- number of diff angles eccentricity
Hyperbola- no of diff angles depending on angle of slice

P value tells you about the asphericity. Tells you how much a surface is steepening or flattening as you go into the periphery. Bigger p= steeper. Lower p= flatter.

Circle/spherical= P=1 as curvature is flattening exactly same in periphery not steepening or flattening

Oblate ellipse- P value greater than 1 as surface steepening in periphery so radius of curvature increases ro large compared to rT and rS in periphery

Prolate ellipse- range of p values between 0-1. Flattening as lower than 1. Further out into periphery radius of curvature increases so surfcae gets more aspheric, flattest away from axis.

Parabola- p value of 0 flattening

Hyperbola- p value of less than 0

Question 73

explain radius for oblate ellipse vs prolate ellipse

Answer 73

In oblate ellipse more steepening so increasing radius of curvature= more steep.

Prolate ellipse- more flattening so as curvature increases the surface is flattening

Question 74

Correcting for off axis aberrations

Answer 74

the lens designer can choose a p value which corrects for a particular off axis aberration. One p value may be used to give minimum tangential error and another may be used to give 0 OAE.

For any tangential error there is a concoid p value that corrects for that error, if the tangential error is positive then a p value of less than 1 must be used so flattening surface in periphery. And if error is negative then p value of more than 1 must be used.

Question 75

Explain the terms in the formulae

Answer 75

Y is the distance from the optical axis
X is the sag of the surface at distance y away from opt axis

R0 is radius at vertex. Increase in this means p is less than 0
Surface astig= tangential power- sag power

Use f= n’-n/r to calculate the power from teh radius. Work out apical radius, sag and tangential radius and then surface astigmatism.

We look at sag to work out centre thickness and edge thickness. Not on equation sheet. A= sag of front surface and B= sag of back surface.

Question 76

Compare surface astigmatism, oblique astigmatism and spherical surface

Answer 76

Surface astigmatism- property of the surface itself, can measure it using a lens measure and power in different meridians. This surface astigmatism counteracts OAE in aspheric lenses when you are looking off axis.

Oblique astigmatism- OAE= tangential- sag power. Work this out via ray tracing through the oblique angle you are interested in.

Spherical surface- don’t get surface astigmatism as radius of curvature is the same in all meridians.

Question 77

Is it good for high plus lenses

Answer 77

Flatter= lower p= thinner lenses. But astigmatism increases with increasing asphericity which is good as the oblique astigmatism can be neutralised with this surface astigmatism. So flatter lens and less OAE. Good for high plus lenses.

Question 78

As we decrease the p value lets talk about the different aspheric surfaces and which ones are the best for what

Answer 78

As we decrease p value OAE increases but FT decreases. The surface astigmatism increases as well which would neutralise this. As P reduces, the difference between the two spherical powers increases so more surface astigmatism as the lens becomes flatter in the periphery.

P1 value is the asphericity of the first lens which is spherical high OAE no surface astigmatism as Frs= Ft. Then -4 barely any distortion as it decreases when lenses get thinner or more aspheric.

0.4 prolate ellipse= not bad lens OAE reduced quite a lot, good MOE of 0.03 (away from 10) good for presbyopes dont have to accomodate.

0.0 parabola- low OAE but higher MOE so its good for non presbyopes who can accomodate, negative MOE its less than 10 so can make image sharp in periphery but not as good for presbyopes.

-4.0 hyperbola- low distortion but not good OAE and MOP is rubbish so wouldnt go for this one

Question 79

What is z value, n value and what angle is oblique astigmatism calculated at

Answer 79

The z value= centre of rotation distance
N= for particular parameters. If changed parameters would have to change design.
Oblique astigmatism is calculated by ray tracing at 35 degrees eye rotation

Question 80

Field pilots for spherical lens, prolate ellipse and parabola

Answer 80

Spherical lens- high levels of astigmatism and MOE need Aspheric lenses to help us with high plus lenses. We want FT and FS to be the same so no distortion. But here tangential increases a lot with eye rotation.

Prolate ellipse- zero MOE as same distance away from BVP so MOP= BVP. Good for presbyopes as no moe so dont have to accomodate in the edges for that sharp image. Lower p so flatter. Aspherics work better w flatter form lens.

Parabola- lens even more aspheric. T ands s values go to the same side here so no distortion. Less OAE as well but MOP is higher so this isnt as good for presbyopes but positive MOE so good for younger pxs who can accomodate to get sharp image at edges.

Question 81

Different aspheric lens forms for aphakia

Low power aspheric lenses facts

Answer 81

conic surface full aperture and lenticular
polynomial surface full aperture and lenticular
blended aspheric lenticulars
zonal aspheric (pseudo aspherics)
aphakics are very uncommon now.

2) lenses less than +6.00D
used to improve cosmetic appearance whilst maintaining good optical performance
cant give better off axis performance than with spherical for low powers
But allows you to use flatter form and lens flattened in periphery as well
now generally on front surface, free form can be on either surface or on both

standard aspheric lenses give greatest benefit for plus lenses
allow us to use flatter lens design (greater apical radius) good optical performance
the lens is further flattened since the sag of the aspheric surface is less than a spherical surface of the same vertex power

Question 82

Aspherics and high index

Answer 82

greater savings in thickness can be gained by using high index materials with aspheric surfaces. The gain is two fold= double thinning

1- reduction in sag of the surfaces as a natural consequence of higher refractive index materials needing less surface curvature to produce the same surface power
2-as the surfaces are flatter, they produce more oblique astigmatism which requires greater asphericity to correct it, this asphericity has the added bonus of added savings in thickness.
(So flatter surface w high index has more off axis astigmatism but this requires a flatter aspheric surface to correct for it)

Question 83

For low powered plus lenses

Answer 83

Top two are best form lenses good moe and OAE. Point focal gives you negative moe which means you’d need to accomodate to make the image shape but presbyopes cant do that so give them Percival instead.

3 is flatter lens but poor optical performance positive moe and high OAE but if you apply an aspheric surface to this flat design good off axis performance again but moe is slightly negative again accomodation but thinner and better performance bit more distortion and neg moe compared to Percival.

For -ve lenses thickness savings not that great

Question 84

Verification of aspheric surfaces

Answer 84

using a lens measure, measure the variation in power across an aspheric surface. So lens power changes in the periphery.

Positive lenses get less positive in the periphery= front surface is aspheric, rear is spherical or toroidal. Less positive again young px can accomodate.

Negative lenses- power increases towards the edge of the lens, front surface is aspheric and rear is spherical or toroidal. (More -ve young can accom)

Spherical- constant power reading on a section through optical axis.

Question 85

Dispensing aspheric lenses

Answer 85

must be carefully centred so wearer is not looking through peripheral astigmatic zones in the primary gaze so do mono pads and heights.

Prescribed prism must not be induced by displacing the lens, can tilt the back surface but must ensure px is still looking through the optical centre.

Question 86

1) Explain the advantages of aspheric lenses for low power lenses-

2) is a pal and a toric lens an aspheric lens? And how to we verify if a surface is aspheric

Answer 86

1) low power lenses less than 6.00D, aspheric surfaces allow for flatter lenses compared to spherical surfaces whilst at the same time giving as good optical performance in the periphery as steeper best form spherical lens. The affect is more noticeable in positive lenses. allows you to use the flat form best form lens and then take care of the astigmatism using the asphericity= flatter surface.

technically it is as it doesnt have a spherical surface but it isnt aspheric in ophthalmic lenses as a PAL is not rotationally symmetrical so does not fit the definition.

same, technically is aspheric as not spherical but not rotationnaly symmetrical.

4) Using a lens measure, if a surface is spherical it does not matter where you place the lens measure, you get the same reading. For an aspheric lens the reading will change as you move the lens measure away from the optical centre. either increases or decreases in the periphery.

Question 87

Explain the values in the table

Answer 87

same back vertex powers but different asphericities for the front surface. Aspheric front surface with different values.
P1 is the asphericity value of the lens surface, p=1 is a spherical surface. 0.4 is a prolate ellipse. 0 is a parabola and -4 is a hyperbola. Values less than 1 means the surface is flattening in the periphery the smaller the number the more it flattens.
OAE is the oblique astigmatic error at the stated given eye rotation and is calculated by ray tracing to find Ft-Fs for the lens.
MOP- is the mean of the saggital and tangential powers and is calculated from Ft+Fs/2
D percent is the percent distortion
F1t and Fgs are the tangential and sagittal surface powers for the front surface which can be used to compensate for the oblique astigmatism calculated in column 2.

Question 88

Calculate moe for each lens and which do you think is the best lens design and why

Answer 88

MOE= MOP- Fv’ so take away 10 for each one. Last 2- accomodate so younger non presbyopic. Last one isnt great lens. Second great for presbyopes

0.4 prolate ellipse- has the smallest moe so good for presbyopes. Slightly more distortion and OAE
0.00 parabola- lowest astigmatism but higher MOE negative moe as less than 10.00 so the wearer could accomodate to make image sharp so good for non presbyopes. Lower distortion as well

Question 89

What does the Field plot tell you about the lens and its optical properties

Answer 89

Back vertex power is +10.00D
the front surface is aspheric- prolate ellipse
the back surface is spherical
The MOE is minimal in the periphery= percival style lens design. An acc Percival lens has two spherical surfaces though
it has low positive levels of OAE about 0.5D at 40 degrees (goes down, so away from true bvp it goes down by about 0.5 either side)
it is a shallow meniscus lens which means it does not have steeply curved surfaces and will be reasonably thin and cosmetically acceptable.

Question 90

Miss h has a rx of : 10.50DS and L: +10.75/-0.75 X 45. She had one pair made up elsewhere but wants a cheap second pair so dispensed her with spherical front surface to her rx on the back surface into a carefully chosen frame to keep the centre thickness to a minimal but she comes back unhappy complaining that her vision is blurred through edges of lens but in first pair could see through the edges. What could the problems be due to- explain it to her.

Answer 90

With spherical surfaces it is not possible to find a best form solution for any of the three lens design philosophies for powers greater than +7.00DS which means her lenses with spherical surfaces wont have good off axis image quality.
Her old lenses may be aspheric which would give better off axis performance, normally put on the front surface of the lens.
you can confirm this using a lens measure as if you have an aspheric surface the power measured changes as you move the lens measure over the surface from the centre to the edge.
keep the explanation simple, explain her old lenses were special aspheric lenses designed to give clear vision over the whole lens which is not possible for her rx with spherical lense