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Image Jump occurs when a patient looks from distance into the add power of their lens. Their perceived image may appear to “jump” due to the induced prism of the bifocal segment. It can be calculated using Prentice’s Rule based on the add power (F) and type of bifocal (d). These are the distances for three types of bifocals:

  • Flat Top 28: 0.50cm
  • Executive: 0.00cm
  • Round: radius of the round segment in centimeters

 Equation to convert from M-Notation to Snellen-Notation

Test distance / M letter size = 20ft / Snellen letter size