4.1.5 Dispenses a range of lens forms to include complex lenses, multifocals and high corrections, and advises on their application to specific patients’ needs.

Question 1

High Minus/Plus:

Lens type & form

Answer 1

• Lens type & form
  
  • High index - 1.74 - better for weight & fit into frame hence better cosmesis
  • Aspheric - better optics, peripheral portion flattened more to reduce edge thickness. High plus means base curve is flattened so less bulbous, better frame fit & less weight
  • High V value reduces TCA. For high index glass or plastic, the v value is quite low (about 30) so patient may struggle with chromatic aberrations, especially when viewing light sources & potentially during night driving

Question 2

High Minus/Plus:

Centration

Answer 2

• Mono-pds & heights - ensures correct OC to prevent prismatic effect to which tolerance from patient won’t be very high!
• Inward decentration - increases temporal edge thickness in high minus & increased nasal edge thickness in plus
• KEEP BOX CENTRED DISTANCE CLOSE TO PD TO AVOID DECENTRATION!
• Poor centration can impact TCA

Question 3

High Minus/Plus:

Frame

Answer 3

• Small H&V eye sizes - lower diameter = lower thickness
• BVD - increases minifying (high minus) & magnifying (high plus). More plus as BVD increases
  
  • Minimising BVD means - better FOV, reduced TCA & distortion, reduced spectacle magnification (cosmetically better for patient)
• Plastic, thicker rimmed frames to reduce effect of edge thickness on cosmesis. Nose pads may be obstructed by the sheer edge thickness itself.
• Ring diplopia (high minus) - overlap of fields at & around the lens causing diplopia towards very periphery but hardly noticed by patients
• Ring scotoma (high plus) - as image is more magnified & converged, potential for scotoma in far periphery so that if a random object suddenly comes in from periphery after being in that blind spot, it can shock the patient. Not likely but using aspheric lens helps avoid this or even a polynomial surface

Question 4

High Minus/Plus:

Coatings (generally)

Answer 4

• MAR coat - reduces power rings from high minus & effect of TCA. Reduces a lot of glare from screens & headlights from cars (especially at night) by increasing light transmission through lens. Reflections completely eliminated. Much better to use for high n.
• Polarising - reduces glare & brightness. Glare from wet surfaces, water, pavements, metal etc
• Transitions - react with UV, becoming tinted & acting as UV filter
• Scratch resistant coating - NOT anti-scratch

Question 5

Varifocal:

Answer 5

• The surface astigmatism of a varifocal (PPL) is proportional to the Add power.
  For example, the unwanted cylinder power of a +3.00 lens is roughly three times the cylinder power of a +1.00 Add lens.
  
  • Advice for patient – The higher the add the more distortion, so keeping with the same design after each eye exam may result in a smaller reading area (assuming add increases after each eye exam, which it will in cases of presbyopia)
• Prism thinning - PPLs prism thinned to reduce thickness & hence cosmesis. Base Up removed once upper edge of lens thinned. This effectively leaves 2/3 Add as Base Down
  PPLs should be checked for prism at PRP. Problems may arise if px has Odd Adds
  
  • Leaves lower centre thickness
• Add - 2 digit number without plus sign or decimal, always below temporal alignment reference marking of varifocal
• Manufacturer mark - manufacturer logo below nasal alignment reference marking of varifocal

Question 6

index ranges?

Answer 6

 Normal index = 1.48 – 1.54 (abbe ~58)
 Mid index = 1.54 to 1.64 (abbe ~37)
 High index = 1.64 – 1.74 (abbe ~32)
 Very high = >1.74 (abbe ~33)

Question 7

 The higher the refractive index

Answer 7

thinner the lens = ability to bend light more efficiently
o Thinner lenses are due to the increased index and flatter radii of curvature

 High index = reduction in abbe number = increased chromatic aberrations
o ABBE number = measurement of the materials dispersion of light
o Chromatic aberration = white light splits into its colour components as the lens is unable to bring various wavelengths to the same focal point
o High abbe = desirable = low dispersion

 MAR included due to increased reflections due to the denser lens material required
 Sometimes the vision can feel better with CR39 compared to high index due to increased off-axis aberrations in high index lenses.

 As refractive index increases density and amount of aberration increases and weight decreases (plastic) or increases (glass)

Question 8

Aspheric lenses

Answer 8

 Conventional lenses = spherical front surface
 Aspheric lenses = complex in that the curvature of the front changes from the centre to the periphery
 There lenses have a flatter base curve, usually 2 – 3D less than standard lenses
 Reduce spectacle magnifications due to reduced centre thickness = better cosmetic appearance
 Minus lenses = back surface aspheric is useful as it allows lens to be fitted closer to the eye given wider FoV
 The curvature of aspheric lenses changes across the surface, allowing for a lighter lens that is often of better optical quality due to reduced off-axis aberrations.
 Heights needed
 Each point has different curvature (different effective OC)

Question 9

Dispensing high index

Answer 9

 Match HCD to PD to reduce need for decentration
 Smaller lens to: reduce thickness, weight
 Ensure frame pd & px pd similar to avoid decentration and increasing thickness
 Smaller blank size for plus

Question 10

BVD calculation:

Answer 10

Power new = Power old/ 1+ (d metres x Power Old)

Question 11

Why BVD should be kept to a minimum

Answer 11

• maximises FOV – if reduced, it becomes an issue, particularly with progressive lenses, because the intermediate and near portions are already relatively narrow and are likely to have been narrowed further by an increased reading addition. So, an increased vertex distance can be a significant source of patient dissatisfaction, even in low powered lenses.
• reduces distortion
• reduces chromatic aberration
• +ve lenses – reduces retinal image with drop in VA
• -ve lenses – increases retinal image with increase in VA
• +ve lenses – reduces spec. mag. of px’s eyes
• -ve lenses – reduces spec. min. of px’s eyes