Worksheet 10: Vertex Distance Changes.

Effective Power, Compensated Power

Questions 1-4: The Rx is -16.50 DS OU, refracted at 10mm. The glasses fit at a vertex distance of 8mm.

  1. If the lenses are ordered -16.50 DS OU, the wearer will experience

    2. too much minus power.
    too little minus power.
  2. The lenses that could be ordered to compensate for this change in vertex distance would be

    3. more minus.
    less minus.
  3. The effective power of the lenses, if ordered -16.50DS, at the vertex distance of 8mm, will be

  4. The lenses that could be ordered to compensate for the change in vertex distance would be

Questions 5-9: The Rx is +18.00-2.75 x 110. This Rx is the result of an overrefraction of the wearer's current glasses, which fit at 12mm vertex distance. The new glasses will fit at 9mm vertex distance.
  1. If the lens is ordered as is, the wearer will experience

    2. too much plus power.
    too little plus power.
  2. The lenses that could be ordered to compensate for this change in vertex distance would be

    3. more plus.
    less plus.
  3. The cylinder power in the compensated prescription will be

    4. more than -2.75.
    less than -2.75.
    -2.75.
  4. The effective power of the lenses, if ordered +18.00 -2.75 x110, at the vertex distance of 9mm, will be

  5. The lenses that could be ordered to compensate for the change in vertex distance would be
  1. The best way to measure the wearer's vertex distance is with

    2. corneal reflex pupilometer.
    a distometer.
    a ruler.
    vertometer.
  2. A person has a strong minus glasses Rx. The contact lens that will give the equivalent vision will be

    3. more minus than the glasses Rx.
    less minus than the glasses Rx.
    always very close to the glasses Rx
  3. A person has a strong plus Rx. This person�s contact lens Rx will be

    4. stronger than the glasses Rx.
    weaker than the glasses Rx.
    always very close to the glasses Rx

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Textbooks:
Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd Ed, Butterworth-Heinemann, 1996
Stoner, Optical Formulas Tutorial, Butterworth-Heinemann, 1997

Copyright 1998 - 2001, Ellen Stoner, MALS, ABOM, NCLC
Registered students may print one copy for study purposes.