Multifocal dispensing

• Bifocals- 2 distinct powers

• Trifocals- 3 distinct powers

• PPLs- gradual change

• Degressive - gradual change

• main lens power and segment

Ads:

• allows focusing at 2 different distances without changing specs

• stable fixed near portion

• no distortion at edges ( PPLs)

Disads:

• abrupt change in power

• visible segment + reflections = not good cosmesis

• area for near may be limited

• Benjamin Franklin 18thC

  • Franklin Split = 2 SV lenses cut and glued together

  • cosmetically not good

• Cemented Bifocals- segmented cemented onto main lens

  • segment = same back curvature as main lens

• Fused Bifocals - depression is ground into main lens and segment bonded into lens.

  • only for crown glass lenses

• Solid Bifocals- made from one piece of material

  • ridge can be felt

  • most common - glass and plastic lenses

• OC of seg = geometric centre of circle that seg is from

  • R seg OC = middle

  • D/C seg= top of circle is cut off so OC is nearer top

• for min prism: Os and On are coincident

• Jump = amt of vertical prism, results in image “jumping” on downgaze

  • depends on seg shape + OC psn

    • greatest for R seg, esp large diamter (base down)

    • less for D seg - (base up)

    • no jump for E seg

• bigger diameter = bigger jump

• all segs visible

• Straight top seg ( D and E) - more visible esp on plastic lenses + ridge felt = reflections

• C seg curved and angled = less noticeable cosmetically + less reflections. ( premium lenses)

• IPD - mono

• seg top position = HCL to lower limbus( should be BELOW HCL)

• Near centration distance not needed

• lens that has more than one power with gradual transition in power btwn distance and near + progressive zone

Ads:

• allows some intermediate vision as well as distance and near

• cosmetically same as SV

Disads

• high add = steeper change in power

• limited FOV esp for intermediate

• have to look down for intermediate and near

• unwanted aberrations at edge of lens

• lines joining points with equal abberational astigmatism

  • closer spacing = more obvious blurred areas

• freeform - uses computer numerically controlled (CNC) lathe for precise curvature

  • CNC has single diamond tipped point

• PPLs have progression on front surface with distance Rx at the back with mould

• lines join points of equal power on lens surface (different to iso cyl plots)

  • shows where add will located

  • shows width and depth of zone for each power

• MFH- centre of fittingcros to bottom of NV circle

• CL- dstance btwn start of progression and top of NV circle

lower add + longer corridor = wide FOV without aberration

if add is above 2.00, do not recommend PPLs

lower plus = shallower curve

higher plus= steeper curve

• short CL for shallow frames

  quicker transition - some px may like to not look too far down for near

  • expense of intermediate

• less need for premium lens when add is low bz aberration will be low. Upgrade px to premium when adds go up

mono IPD has to cincide with fitting cross- FC on pupil centre

changing panto tilt: increase = brings NV area up, vice versa

leave markings on

check FC coincides with pupil centre

distance and near circles in frame

check distance is clear

re-advise px of aberrations

for new wearers:

• explain that eyes need to move for diff zones, not head + move head when looking side to side