Extra Questions for Module 11
Back and Front Vertex Power
Answers

Positional Effective Power Worksheet
  1. Due to slippage, an aphakic patient's eyeglasses (+10.00 OU) have moved 5 mm forward.  What is the effective power of the eyeglasses at the new position?   +10.53

    2.  
  2. During an adjustment, and optician moved a patient's -12.50 D OU lenses 3 mm closer to the patient's face.  What is the effective power of the lenses at the new position?  -12.99

    3.  
  3. What is the PEP of:
      4.  
    1. +16.00 lenses are moved from a vertex distance of 11 mm to 6 mm? +14.81

      2.  
    2. -11.00 lenses are moved form a vertex distance of 8 mm to 12 mm? -10.54

      3.  
    3. -4.50 is moved from 16 mm to 10 mm?  -4.62

      4.  
    4. -10.75 is moved from 7 mm to 14 mm? -10.00

      5.  
    5. +18.00 is moved from 17 mm to 13 mm?  +16.79

      6.  
    6. -15.00 is moved from 15 mm to 10 mm?  -16.22

      7.  
    7. -11.50 is moved from 11 mm to 14 mm?  -11.12

      8.  
    8. +12.00 is moved from 16 mm to 12 mm?  +11.45

     
  4. If a plus lens is moved away from the eye, will it have to be more plus or less plus to give the wearer the effect the Dr expected?  Will have to be ordered with less plus power.

    5.  
  5. If a minus lens is moved away from the eye, will it have to be more minus or less minus to give the wearer the effect the Dr expected?  Will have to be ordered with more minus power.

    6.  
  6. Will a contact lens for a myope have more power or less power than the person's glasses? CL will have less minus power than the glasses. Why?  Because the CL is closer to the eye than the glasses are, so the CL will be effectively stronger than the same glasses lens.

    7.  
  7. Will a contact lens for a hyperope have more power or less power than the person's glasses? CL will have more plus power than the glasses. Why?  Because the CL is closer to the eye than the glasses are, so the CL will be effectively weaker than the same glasses lens.


Vertex Power Worksheet
Compensate for the effective power changes if . . .
  1. the lens moves closer to the eye . . .
      2.  
    1. -19.50 moves 8 mm  -16.87

      2.  
    2. -7.00 moves 2 mm  -6.90 rounds to -6.87
    3. +19.00 moves 2 mm  +19.75

      4.  
    4. +11.50 moves 7 mm  +12.51 rounds to +12.50

     
  2. the lens moves away from the eye . . .
      3.  
    1. -12.25 moves 4 mm  -12.88 rounds to -12.87

      2.  
    2. +8.75 moves 5 mm +8.38 rounds to +8.37

      3.  
    3. +13.75 moves 9 mm  +12.24 rounds to +12.25

      4.  
    4. -14.75 moves 9 mm  -17.01 rounds to -17.00
Vertex Power Worksheet
  1. The Rx reads +13.50, refracted at 12 mm.  The new frame sits at 14 mm.  What should the dispenser order?   +13.15 rounds to +13.12

    2.  
  2. If the refracted vertex distance is 12 mm and the glasses will be worn at 7 mm, what should be ordered for a -11.75 -2.50 x180 Rx?
      3. -11.75 becomes -11.10, rounds to -11.12
      -14.25 becomes -13.30
      difference between -11.12 and -13.30 is -2.18, rounds to -2.12
      Rx becomes -11.12 -2.12 x 180 (will accept -11.12 -2.25 x 180)
  3. Recompute for frames sitting at 14 mm, if the refracted vertex distance is 15 mm:  +9.75 -0.75 x 090.
      4. +9.75 becomes +9.85, rounds to +9.87
      +9.00 becomes +9.08
      difference between +9.87 and +9.08 is -0.79, rounds to -0.75
      Rx becomes +9.87 -0.75 x 090
  4. The wearer's old frames have a vertex distance of 8 mm.  The new, big, 'fashion' sunglasses have a vertex distance of 13 mm.  What will you order?  Rx:  +12.50 -4.75 x125.
      5. +12.50 becomes +11.76, rounds to +11.75
      +7.75 becomes +7.46.
      difference between +11.75 and +7.46 is -4.29, rounds to -4.25
      Rx is now +11.75 -4.25 x 125

The answers are here.  Please do them yourself before you go to the answers.  If you understand it all and you don't need the practice then it will only take you a few minutes to do them.  If you are struggling with the formulas do one or two, check them, do a few more, check them, . . . just make sure that you do not look ahead of the ones that you did and are checking.

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