4.1.1

Question 1

How do you calculate vertical differential prism

Answer 1

P= C x F so times sph of one eye with decentration(0.5) and do the same to other eye and the difference between the two is the vertical differential prism

Question 2

What are the tolerances for differential prism

Answer 2

0.25-0.5 = acceptable/minimal discomfort
0.5-1.0 = mild adaptation, noticeable symptoms like eye strain/HAs
>1 = discomfort/DIP/eye strain and req gradual adaptation

Question 3

How do you check lens thickness without an order form

Answer 3

Calipers with a display screen, position caliper jaw on surface of lens at centre to measure central thickness

Question 4

What is VDP and HDP

Answer 4

VDP is vertical differential prism when optical centres are not aligned vertically and HDP is horizontal differential prism when optical centres are not aligned horizontally

Question 5

What condition causes vertical imbalance when switching from the near portion?

Answer 5

Anisometropia

Anisometropia refers to a condition where the two eyes have unequal refractive power.

Question 6

What can the difference in lens powers in anisometropia lead to?

Answer 6

Vertical disparity

Vertical disparity can result in symptoms such as double vision or discomfort.

Question 7

What is used to correct the vertical imbalance in near portion of the lens?

Answer 7

Slab off

Slab off is a technique used in lens design to accommodate differences in prescription between the two eyes.

Question 8

In which eye is the slab off prism correction applied?

Answer 8

The more myopic eye

Myopia refers to nearsightedness, where distant objects appear blurry.

Question 9

Where does the prism correction start in bifocals?

Answer 9

At the dividing line between dv and nv

‘dv’ refers to distance vision and ‘nv’ refers to near vision in bifocal lenses.

Question 10

In bifocals, where is the prism correction present?

Answer 10

Only in the nv

The near vision (nv) portion is designed to assist with close-up tasks.

Question 11

Fill in the blank: Anisometropia can cause an imbalance in the amount of ______ experienced.

Answer 11

prism

Prism refers to the degree of deviation of light entering the eye.

Question 12

What is monocular centration

Answer 12

Align each lenses optical centre to the pupil of each eye by measuring the distance from centre of patients nose to pupil of each eye separately. This is crucial if eyes are not symmetrically placed

Question 13

What is the relationship between good VAs and anisometropia?

Answer 13

Good VAs are more likely to cause anisometropia

Anisometropia refers to a condition where the two eyes have a significant difference in refractive power, which can lead to visual discomfort and difficulties in coordination.

Question 14

Why are bad VAs less likely to cause anisometropia?

Answer 14

The brain suppresses the image from the poorer eye

This suppression can help to mitigate the effects of unequal visual input from the eyes.

Question 15

What occurs with large anisometropia and good monocular VAs?

Answer 15

Anisocoria occurs

Anisocoria is a condition where there is a difference in the diameter of the pupils, which can be related to differences in image size or magnification between the eyes.

Question 16

How does power difference between the eyes affect prismatic effect?

Answer 16

Each 1D in power difference induces 1D of vertical differential prismatic effect

This effect is notable at the near viewing point (NVP), typically located 8-10mm below the optical centers of the lenses.

Question 17

What are common problems for anisometropia/anisokonia

Answer 17

Vertical differential prism
Horizontal decentration
Large change in rx effecting BV status
Frame size change (smaller to bigger)
Frame shape change(more bowing effect)

Question 18

Solutions for anisometropia/anisokonia

Answer 18

Reduce rx to help adaptation
Prescribe prism - calc amount of vertical decentration and it’s over 1D and disp is multis then check if the compensating prism makes the vision more comfortable

Question 19

What do the following frame markings mean = 47.21 145
And how would u calculate frame OC

Answer 19

47 = lens diameter
21 = bridge width
145 = arm length
Frame OC = 47+ 21
= 68 /2
= 34

Question 20

Why does changing to higher index lens also require adaptation

Answer 20

Higher index lenses have more chromatic aberrations (different wavelengths of light are refracted at diff angles)

Question 21

What are the symptoms of Aniseikonia?

Answer 21

Visual discomfort, visual distortion, difficulty with stereopsis/BV, dizziness, H/A, nausea

H/A stands for headache.

Question 22

What type of lens is used for single vision dispensing for an anisometropic patient?

Answer 22

Aspheric lens for the most positive powered eye

The aspheric lens reduces retinal image size by altering lens curvature to control magnification

Question 23

How does an aspheric lens affect the retinal image size?

Answer 23

The retinal image size is reduced as the aspheric surface is approximately 2.00D flatter

This flattening helps in managing Aniseikonia.

Question 24

What is the purpose of an Iseikonic lens?

Answer 24

To increase the spectacle magnification of the least positive eye

This is achieved by increasing the base curve.

Question 25

How is the thickness of an Iseikonic lens adjusted?

Answer 25

By increasing the base curve to approximately 8.00D ## Footnote This adjustment helps to accommodate the least positive eye.

Question 26

What is the formula for Spectacle Magnification?

Answer 26

Spectacle Magnification = Shape Factor x Power Factor ## Footnote This formula is essential for understanding lens design in Aniseikonia.

Question 27

Define Shape Factor in the context of lenses.

Answer 27

A ratio of surface curvatures that describe the shape of the lens, typically ranging from -2 to +2 ## Footnote Shape Factor influences how the lens shapes the image.

Question 28

What does the Power Factor represent in lens design?

Answer 28

The dioptric power of the lens considering the power and vertex distance of the lens ## Footnote Power Factor plays a crucial role in determining lens effectiveness.

Question 29

What is slab off prisms?

Answer 29

A method of grinding the lens to add a specific amount of prism to one lens to compensate for the difference in prescription between the two eyes.

Question 30

In what types of lenses is slab off used?

Answer 30

Bifocals and varifocals.

Question 31

Where is the slab off added in the lens?

Answer 31

To the lower part of the stronger lens.

Question 32

What effect does looking down have for an anisometropic patient?

Answer 32

It causes a vertical prismatic effect.

Question 33

What symptoms can slab off prevent?

Answer 33

DIP/eyestrain.

Question 34

How is slab off calculated?

Answer 34

By halving the difference between the two eyes and adding to the most negative lens.

Question 35

Why is the most negative lens used in slab off calculation?

Answer 35

It induces the most base down (BD) prism.

Question 36

What does base down (BD) prism correct?

Answer 36

Image alignment when a person looks down.

Question 37

What are polarizing lenses designed to reduce?

Answer 37

Glare ## Footnote Polarizing lenses filter out specific orientations of light waves.

Question 38

What do polarizing lenses contain that blocks certain types of light?

Answer 38

A polarizing film made from polymer or iodine crystals. The material is stretched to align the crystals in a vertical direction to help block out the horizontal light

Question 39

How do light waves travel?

Answer 39

In waves, vibrating in many directions ## Footnote Light from the sun scatters in all directions.

Question 40

What happens to light when it reflects off flat surfaces?

Answer 40

It becomes polarized ## Footnote This means the light waves align mostly in a horizontal direction.

Question 41

What type of light do polarizing lenses allow to pass through and how does this make the glasses polarised

Answer 41

Only Vertically oriented light can pass through Natural light travels in all directions but when it hits a flat surface it reflects off at a horizontal angle causing horizontal polarised light which causes intense glare

Question 42

What does the 'D' in the D28 segment refer to?

Answer 42

The shape of the near-vision portion ## Footnote It is D-shaped.

Question 43

What does the '28' in D28 segment refer to?

Answer 43

The width of the segment in millimeters ## Footnote In this case, it is 28 mm wide at its widest point.

Question 44

Where is the D-shaped segment typically placed in the lens?

Answer 44

Towards the bottom of the lens ## Footnote The flat side of the 'D' faces upwards.

Question 45

What is one characteristic that differentiates the C segment from the D segment?

Answer 45

Smaller diameter of 22mm ## Footnote The C segment has a more discrete appearance.

Question 46

What is a preferred use for the C segment?

Answer 46

More specific tasks requiring a smaller reading area ## Footnote It has a round topped design.

Question 47

What is aniseikonia?

Answer 47

A condition where there is a significant difference in the perceived size of images between the two eyes

Question 48

What causes aniseikonia?

Answer 48

It can occur when the refractive power or shape of the lenses in each eye differs

Question 49

What are anisekonic glasses used for?

Answer 49

They are used for correction of aniseikonia

Question 50

How do anisekonic glasses work?

Answer 50

The lenses change how big or small the images look when viewed through them

Question 51

What is the effect of wearing anisekonic glasses?

Answer 51

They help make the sizes look more similar between both eyes