PPLs

• PPL


  lens with at least one progressive surface that provides more positive power as the wearer looks down
• aberrations

  1. Deviation of light rays that results in a blurred/imperfect image
  2. light rays from an object dont converge/ diverge at the same point
• 'swim'

  1. Distortion in the periphery especially when moving around, worse if lenses are fitted poorly
• Hard design VF

  1. full width distance portion
  2. narrow intermediate
  3. wide reading area
• Soft design VF

  1. wider intermediate
  2. distance and near portions are affected by the intermediate portion

  
• iso cylinder

  1. lines that each represent 0.5DC of unwanted astigmatic error in varifocals
  2. soft designs push isocylinders and distortion to the periphery
• Aberrations experienced in hard designs

  1. more levels of aberrations but in smaller areas which leads to the smaller intermediate area
  2. makes the design hard to get used to because modern life incorporates significant intermediate use
• Aberrations experienced in soft designs

  1. wider intermediate area means the level of aberrational astigmatism is less as its more spread out
  2. some aberrational astigmatism in the distance and near
  3. unwanted aberration is at a lower level than with harder designs, it is less noticeable and easier to tolerate, and as a result, soft designs are easier to adapt to than hard ones.
• what is corridor length most dependant on

  the frame
• What does a shorter corridor mean

  1. higher add
  2. 'harder' lens design
  3. more peripheral aberrations
• what does a longer corridor mean?

  1. better to adapt to
• where is the reference point for the fitting of varifocals

  the fitting cross is aligned with the patients centre of pupil
• importance of PPL lens markings

  1. lenses are identifiable
  2. correct orientation when glazing
  3. subsequently can be checked and verified
• what is prism thinning

  The removal of BASE UP prism to incorporate base down prism to allow for a thinner and more equal thickness lens

  
• why do we do prism thinning

  1. to thin lenses
  2. equalises edge profiles at the top and bottom of the lens (cosmetics)
• what would be the result of no prism thinning

  A thicker top edge profile as PPLs have more curvature at the bottom
• why do PPLs have more curvature at the bottom

  This is where the reading ADD is situated which is a positive, convex part of the lens.
• How to calculate prism thinning

  2/3 X reading add
• What does a high(er) ADD result in?

  • more peripheral aberrations
  • lens becomes more harder which means visual zones are narrower and harder to adapt to
• advantages of PPLs

  1. convenient
  2. full range of working distances
  3. cost effective (sometimes)
  4. cosmetics
  5. no jump
• Disadvantages of PPLs

  1. narrow/ low intermediate leads to scanning=postural changes and possible injury
  2. price
  3. non standard working distances e.g. pilots and mechanics
  4. peripheral fixation (crossing roads, driving at junctions) requires scanning so further adaptation required
  5. anisometropic pxs experience differential prism when fixating away from the OC