high prescription dispensing

Question 1

what is a high prescription ?

Answer 1

1. British standards states that BVD has to be part of the prescription when Rx>+/-5.00D (BS 2738-3)
2. high index lenses often suggested for Rx>+/-4.00D

Question 2

what do should lenses provide ?

Answer 2

. should provide good vision ( FOV, aberrations, Rx)

. should be cosmetically acceptable to the patient

. should be comfortable

Question 3

what does a higher index mean ?

Answer 3

. higher index=worse vision

Question 4

what is the effect of lens thickness on lens measurements?

Answer 4

. if lens is thick , we need to increase the BVD so the lens isn’t touching the patients lashes when they are blinking

Question 5

what does providing a lens that has good vision , cosmetically acceptable and comfortable depend on ?

Answer 5

to satisfy the lenses providing good vision , being cosmetically acceptable and being comfortable depends on

• form of the lens
• material of the lens
• surface treatments of the lens

Question 6

what does the form of the lens tell us ?

Answer 6

• tells us the curves that we’re using on F1 and F2 to prescribe the prescription and we know that these can influence the overall thickness and vision that the patient gets

Question 7

what does the material of the lens depend on ?

Answer 7

• the material of the lens we select will influence the thickness, the minimum centre and edge thickness and the weight or specific gravity

Question 8

what does the surface treatment of the lens affect ?

Answer 8

• the surface treatment of the lens can affect the vision and the durability of the lens

Question 9

where is vision best ?

Answer 9

• vision is best at the optical centre and along the optical axis

Question 10

why has best form lenses been developed ?

Answer 10

• we know the eye doesn’t stay at the optical axis where vision is best and we want the vision off-axis to be as close as possible to optical axis

Question 11

how can the form of the lens influence vision ?

Answer 11

• curves could aim to remove a particular aberration - aspherics
• lens should be kept as close as possible to existing lens form
• minus base toric for minus lenses, plus base toric for plus lenses
• take into account thick lens equation and BVD

Question 12

how can the form of the lens influence cosmesis ?

Answer 12

• steeper curve- results in a thicker lens overall
• form of the lens can alter the minimum centre thickness
  ( thicker centre = thicker lens )
• aspherics ( flatter and thinner)
• appearance through the lens

Question 13

how can the form of the lens influence comfort ?

Answer 13

• larger base curve more difficult to glaze and require a curved frame

Question 14

how does the material of the lens impact vision ?

Answer 14

• v value of lens material

- can influence lens curves and therefore reflection

Question 15

how does the material of the lens impact cosmesis ?

Answer 15

• reflections

- lens thickness- reduced with higher index lenses - increases reflections and aberration

Question 16

how does the material of the lens impact comfort ?

Answer 16

• specific gravity
• thickness- means a heavier lens, difficult to adjust the frame
• can calculate the overall thickness of lens by using sag equation
• safety

Question 17

how do surface treatment of the lens affect vision ?

Answer 17

• vision can be improved with treatment such as antireflection coating ( MAR)

Question 18

how do surface treatment of the lens affect cosmesis ?

Answer 18

• cosmesis can be altered depending on MAR
• tints/mirror coating
• by blocking certain wavelength can improve cosmesis

Question 19

what to consider with mid to high myopes ?

Answer 19

• considerations such as material and an appropriate frame choice can make difference to dispense
• high refractive index - can reduce the thickness
• plastic frame - overall weight of the lens will be less
• decentration of a lens within the frame can result in uneven thickness and a greater thickness on at least one side usually temporal side

Question 20

what to consider with very high minus ( -14.00 and above ) ?

Answer 20

• use a reduced aperture lens
• available as lenticular or blended lenticular ( only a small section of the lens which is used for seeing and contains optimum prescription )

Question 21

what are lenticular or blended lenticular ?

Answer 21

• only a small section of the lens which is used for seeing
• eliminate edge thickness
• easier to glaze
• manufactured by solid design, bonded or hand edged

Question 22

how does a solid lenticular design look like ?

Answer 22

• will have a ridge at the back between the aperture( bit of lens patient is looking through)
  and the carrier ( vision is poor in the carrier )

Question 23

what does the size of the aperture depend on in lenticular ?

Answer 23

• the size of the aperture is dependent on the lens which is chosen
• it differs between lenses and manufactures
• flattened full aperture lenses - 30 mm or 34 mm

Question 24

what are the different apertures for high minus lenses ?

Answer 24

• oval aperture- can be created by using a toroidal rather than spherical surfacing tool to flatten the concave surface
  . can look better than round
• profile aperture- can look better with unusual frame
  . aperture goes round about 3-4mm from frame edge so carrier is closer to frame edge
  . eyes should sit directly within centre of frame
• round aperture -

Question 25

what are blended lenticular ?

Answer 25

blended - aperture curve is blended with the marginal curve with a convex curve, can be using freeform technology in plastic - cosmesis - improved - vision - blending of the margin is highly astigmatic , causes a reduction of filed of view - minus lenticular lenses are available from a range of manufacturers including Norval - minus lenticular lens are available in range of materials - minus lenticular lens are available in transition and multifocal design

Question 26

what happens in high positive lenses ?

Answer 26

- a prescription in the aperture of the lens is highly positively and there is a margin where this lens transitions into the carrier lens which can contain some prescription or afocal area

Question 27

what to consider with high positive lenses ?

Answer 27

. poor field of view leads to a ring scotoma and a jack in the box affect at the margin . reduced aperture lenses are available . available as button, blended or polynomial . available as bifocals and progressives . should be UV400 coated . consider frame fit and size

Question 28

what are button lenticular lenses ?

Answer 28

. traditional high positive lenticular lenses . small positive lens stuck on to a carrier lens - available in sizes where aperture is 30-40mm and can get them in a round bifocal segment

Question 29

what is zonal aspherics blended margins ?

Answer 29

. high positive lenticular lens . better cosmetically but poor vision through area of blending

Question 30

what is polynomial blending ?

Answer 30

. high positive lenticular lens . better cosmetically and better vision - no aberrational astigmatism - eliminates roving ring scotoma and jack in box effect - less head movement

Question 31

how to alter Rx for BVD ?

Answer 31

use step along | F^2/1000

Question 32

what do we need to take into account with near vision use ?

Answer 32

- need to take into account a near vision effectivity error - need to calculate at near as light not from infinity - need to take into account the different performance from the thick trial lens form to a thick spectacle lens - results in lenses being under plussed . NVEE= (t/n)L1(L1+2F1)

Question 33

which patients have a larger field of view ?

Answer 33

myopic px - larger field of view

Question 34

how is field of view calculated ?

Answer 34

- can be calculated by . tan θ = y(37-F)/1000 - where θ is half the field of view - effected by Rx- myopes have an increased FOV, hypermetropes smaller -effected by radius of curvature - steeper curves decrease the FOV - vertex distance - decreased increases the FOV

Question 35

how to calculate spectacle magnification ?

Answer 35

SM= (1/1-dF'v)) x (1/(1-t/nF1))

Question 36

how to reduce spectacle magnification ?

Answer 36

- thinner lens and a flatter front curve will reduce spectacle magnification

Question 37

what to consider when fitting high prescription lenses ?

Answer 37

- consider BVD/pantoscopic tilt - consider decentration ( reduce decentration ) - monocular PDs and heights

Question 38

what is the best way to fit a lenticular or high prescription ?

Answer 38

- get the patient to pick the frame first and adjust the frame and set the trial frame up to the chosen frame characteristics - adjust frame to match trial frame

Question 39

how do frames affect vision ?

Answer 39

- vision can be affected by frame regards to field of view

Question 40

how do frames affect cosmesis ?

Answer 40

- decentration - power rings - minimum blank size

Question 41

how do frames affect comfort ?

Answer 41

- weight of frame - frame should be symmetrical - frame material - size of bearing surface