20 Diagnostic Fitting Practice Questions & Answers

1. 1. A diagnostic GP lens with a base curve of 42.75 D and a power of -2.50 D is applied. After the lens settles, the spherical over-refraction is -0.50 D. You decide to order the lens 0.50 D steeper than the diagnostic lens. What power should be ordered?

   • A.-3.00 D
   • B.-2.50 D
   • C.-3.50 D✓ Answer
   • D.-4.00 D

   The over-refraction is folded in first: -2.50 D combined with -0.50 D gives -3.00 D. Steepening the base curve by 0.50 D creates a plus-powered tear lens behind the contact lens, so the rule Steeper Add Minus requires a further -0.50 D, giving -3.50 D.

   Source: Bennett & Henry, Clinical Manual of Contact Lenses — lacrimal (tear) lens power and the SAM/FAP ruleReport a problem with this question

2. 2. A diagnostic GP lens has a base curve of 7.80 mm and a power of +2.00 D. You refit the patient with a base curve of 7.70 mm. Using the conversion 0.10 mm = 0.50 D, what power should be ordered so the total correction is unchanged?

   • A.+2.25 D
   • B.+2.50 D
   • C.+1.75 D
   • D.+1.50 D✓ Answer

   A smaller radius in millimetres is a steeper curve, so 7.70 mm is 0.10 mm (0.50 D) steeper than 7.80 mm. A steeper base curve creates a plus tear lens, so Steeper Add Minus applies: +2.00 D - 0.50 D = +1.50 D.

   Source: Bennett & Henry, Clinical Manual of Contact Lenses — base curve expressed in millimetres versus dioptres and the SAM/FAP ruleReport a problem with this question

3. 3. Keratometry reads 43.00 D (7.85 mm) at 180 and 44.50 D (7.58 mm) at 090. Because the two principal meridians differ by more than 0.20 mm, how should the starting base curve of a spherical GP lens be selected?

   • A.Fit one half of the corneal toricity flatter than the flattest meridian, then add minus.
   • B.Fit exactly on the flattest meridian, then add plus power for the tear lens.
   • C.Fit exactly on the steepest meridian so the lens vaults the corneal apex.
   • D.Fit one quarter of the corneal toricity steeper than the flattest K meridian.✓ Answer

   Fitting 'on K' is appropriate only when the two meridians differ by about 0.20 mm or less. With greater toricity the lens would rock on the flat meridian, so the base curve is placed steeper than the flattest K by roughly one quarter of the corneal cylinder, which spreads the bearing more evenly across the cornea.

   Source: Bennett & Henry, Clinical Manual of Contact Lenses — initial GP base curve selection, 'on K' versus mean KReport a problem with this question

4. 4. A patient's spectacle prescription is -5.00 -0.50 x 180 at a 12 mm vertex distance. A spherical GP lens is to be fitted on K. What lens power should be ordered?

   • A.-5.00 D
   • B.-4.75 D✓ Answer
   • C.-5.25 D
   • D.-4.50 D

   A spherical GP fitted on K produces a plano tear lens, so the spectacle cylinder is dropped and only the sphere is carried across. At -5.00 D the power must be vertex compensated, and moving a minus lens closer to the eye means less minus is needed, giving -4.75 D at the corneal plane.

   Source: Mannis & Zadnik, Contact Lenses in Ophthalmic Practice — vertex distance compensation of contact lens powerReport a problem with this question

5. 5. How is the initial overall diameter of a soft lens chosen from the prefitting measurements?

   • A.Equal to the horizontal visible iris diameter, so the edge sits right at the limbus.
   • B.Set independently of the visible iris diameter, since the base curve alone controls coverage.
   • C.Larger than the horizontal visible iris diameter, so the lens overlaps the limbus.✓ Answer
   • D.Smaller than the horizontal visible iris diameter, so the lens rides on the cornea.

   A soft lens is designed to cover the whole cornea and to land on the conjunctiva beyond the limbus, so the diameter must exceed the horizontal visible iris diameter. An edge that sits on or inside the limbus produces limbal irritation, poor centration and fluctuating vision.

   Source: Efron, Contact Lens Practice — soft lens diameter selection and corneal coverageReport a problem with this question

6. 6. Which set of slit lamp findings indicates an acceptable soft lens fit?

   • A.Complete coverage, no movement on the blink, and firm resistance to push-up.
   • B.Full coverage, marked lag on upgaze, and edge stand-off in the periphery.
   • C.Edge exposure at the limbus, brisk lag on upgaze, and temporal decentration.
   • D.Full corneal coverage past the limbus, slight movement on blink, and easy recentering.✓ Answer

   An acceptable soft fit needs complete coverage with the edge on conjunctiva, a small amount of movement with each blink so tears can exchange under the lens, and a lens that slides on push-up and then recentres. No movement with firm push-up resistance is a tight fit, while marked lag and edge stand-off is a loose one.

   Source: Efron, Contact Lens Practice — assessment of soft lens fit: coverage, movement and the push-up testReport a problem with this question

7. 7. A soft lens wearer is comfortable at insertion, but by late afternoon the eyes are red, the lens barely moves on blink, and the conjunctiva shows indentation at the lens edge. What is the problem, and what is the correct change?

   • A.The lens edge lifts excessively; reduce thickness and repolish the edge.
   • B.The lens fits too tightly; order a flatter base curve or a smaller diameter.✓ Answer
   • C.The lens is adapted correctly; the wearer simply needs a longer adaptation period.
   • D.The lens fits too loosely; order a steeper base curve or a larger diameter.

   Comfort that decays through the day with increasing redness, absent movement and conjunctival indentation is the classic tight (steep) soft lens picture: the sagittal depth exceeds that of the eye, the edge compresses conjunctival vessels and tear exchange stops. Reducing sagittal depth with a flatter base curve or a smaller diameter restores movement.

   Source: Efron, Contact Lens Practice — the steep (tight) soft lens fit and its managementReport a problem with this question

8. 8. A soft lens wearer reports that vision is sharp just before a blink and blurs right after each blink; keratometer mires over the lens distort immediately after the blink and clear as the eye is held open. What does this describe?

   • A.A tight, steep lens that the lid drapes and that then vaults the corneal apex.
   • B.A loose, flat lens that the lid displaces and that then settles between blinks.✓ Answer
   • C.Residual corneal astigmatism that no spherical over-refraction will ever neutralize.
   • D.A well-centered lens with normal movement that needs no change of parameters.

   In a flat, loose fit the lid grips the mobile lens and shifts it on the blink, so vision and the keratometer reflex degrade immediately after the blink and improve again as the lens re-centres between blinks. The opposite timing, blur before the blink that clears afterwards, points to a tight lens.

   Source: Efron, Contact Lens Practice — vision and keratometry findings in flat versus steep soft lens fitsReport a problem with this question

9. 9. Fluorescein under a GP lens shows a bright pool of dye centrally, a ring where the dye thins to touch in the mid-periphery, and only a narrow band of edge lift. What does this pattern mean?

   • A.The overall diameter is too small, so the optic zone lifts off the apex.
   • B.The base curve is flatter than the cornea, bearing directly on the corneal apex.
   • C.The base curve aligns with the cornea across essentially every meridian.
   • D.The base curve is steeper than the cornea, vaulting the apex with mid-peripheral bearing.✓ Answer

   Fluorescein pools and glows brightly wherever there is clearance between lens and cornea, and thins to a dark zone wherever the lens bears on tissue. Central pooling with a mid-peripheral bearing ring and reduced edge lift is therefore apical clearance, meaning the base curve is steeper than the cornea it sits on.

   Source: Bennett & Henry, Clinical Manual of Contact Lenses — fluorescein pattern interpretation in rigid lens fittingReport a problem with this question

10. 10. Fluorescein under a GP lens shows dark central touch, heavy pooling in the periphery and excessive edge lift; the lens rides low and moves more than expected. What change is indicated?

    • A.Flatten the base curve to reduce sagittal depth and free the lens to move.
    • B.Widen the peripheral curves to raise edge lift and increase tear exchange.
    • C.Steepen the base curve to increase sagittal depth and improve centration.✓ Answer
    • D.Reduce the overall diameter so the upper lid can carry the lens higher up.

    Dark central touch with peripheral pooling and excessive edge lift is a flat fitting relationship, and the low position with excessive movement follows from too little sagittal depth. Steepening the base curve increases the vault, lifts the lens off the apex and settles it into better centration with less movement.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — troubleshooting the flat rigid lens fitting relationshipReport a problem with this question

11. 11. A GP lens fits acceptably. You increase the overall diameter by 0.5 mm and leave the base curve unchanged. What happens to the fit, and what compensates for it?

    • A.Sagittal depth is unchanged, since only the base curve sets the lens vault.
    • B.Sagittal depth increases and the fit tightens; flatten the base curve to compensate.✓ Answer
    • C.Sagittal depth increases but the fit loosens; widen the peripheral curves.
    • D.Sagittal depth decreases and the fit loosens; steepen the base curve instead.

    Sagittal depth is set by base curve and diameter together, so extending the same curve over a wider chord raises the vault and tightens the fitting relationship. Flattening the base curve lowers the vault again, which is why diameter and base curve are always changed as a compensating pair.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — sagittal depth relationships between base curve and diameterReport a problem with this question

12. 12. A GP lens is slightly tight. Which modification can be carried out on the existing lens in the office rather than requiring a new lens?

    • A.Steepening the back optic zone radius, which also loosens the fit somewhat.
    • B.Increasing the center thickness of the lens, which loosens it even more.
    • C.Increasing the overall diameter of the lens, which likewise loosens it.
    • D.Widening and blending the peripheral curves, which loosens the fit.✓ Answer

    Every in-office modification removes material and therefore flattens or loosens the fitting relationship, and widening and blending the peripheral curves is one of the few changes a lathe and polishing tools can make to a finished lens. A steeper radius, a larger diameter or a thicker lens all add material and all tighten the fit, so each requires a new lens.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — in-office modification of rigid lensesReport a problem with this question

13. 13. Keratometry reads 43.00 D at 180 and 43.50 D at 090. Which combination of specifications would most closely produce a superior lid-attachment GP fit?

    • A.Base curve considerably steeper than steepest K, with a larger overall diameter.
    • B.Base curve slightly steeper than flattest K, with a smaller overall diameter.
    • C.Base curve slightly flatter than flattest K, with a larger overall diameter.✓ Answer
    • D.Base curve exactly on flattest K, with the smallest diameter available.

    A lid-attachment design must ride high enough for the upper lid to hold the lens edge through the blink. A base curve flatter than K lowers sagittal depth so the lens is not held down on the apex, and a larger diameter gives the lid a superior edge to engage. On-K or steeper curves with small diameters give an interpalpebral lens instead.

    Source: Mannis & Zadnik, Contact Lenses in Ophthalmic Practice — lid attachment versus interpalpebral rigid lens designsReport a problem with this question

14. 14. A GP lens ordered as a lid-attachment design rides low, sitting below the pupil, and is never captured by the upper lid; fluorescein shows central pooling. What is the best change?

    • A.Flatten the base curve and enlarge the lens so the upper lid can catch the edge.✓ Answer
    • B.Add a minus carrier lenticular to thin the edge, keeping curve and diameter as ordered.
    • C.Keep every parameter and coach the patient to blink more completely through the day.
    • D.Steepen the base curve and shrink the lens so it rides inside the palpebral aperture.

    Central pooling means the lens is steeper than the cornea, and the excess apical vault is holding it down away from the upper lid. Flattening the base curve reduces that vault, while a larger diameter carries the edge up under the lid so the blink can lift and hold the lens in the intended superior position.

    Source: Mannis & Zadnik, Contact Lenses in Ophthalmic Practice — rigid lens positioning and the low-riding lensReport a problem with this question

15. 15. A soft toric lens was ordered with a cylinder axis of 090. At the slit lamp the scribe mark sits 15 degrees to the examiner's left of its intended position, and the rotation is the same after every blink. Applying LARS, what axis should be ordered?

    • A.Axis 105✓ Answer
    • B.Axis 075
    • C.Axis 120
    • D.Axis 090

    LARS stands for Left Add, Right Subtract, and the direction is judged from the examiner's view of the patient. A mark displaced 15 degrees to the examiner's left means 15 degrees is added to the axis that was ordered, so 090 becomes 105.

    Source: Efron, Contact Lens Practice — soft toric lens rotation and the LARS ruleReport a problem with this question

16. 16. A soft toric lens rests 20 degrees away from its ordered axis, but returns to exactly the same position after every blink and after each push-up. What is the best next step?

    • A.Compensate the ordered axis by the measured rotation and reorder the same design.✓ Answer
    • B.Order the same lens again and let the wearer adapt over the next two weeks.
    • C.Change the stabilization design, since any rotation is unacceptable.
    • D.Add 0.50 D of minus power to sharpen the blur caused by the rotation.

    What matters in a toric evaluation is repeatability rather than the size of the rotation. Rotation that returns to the same place after blinking and push-up is stable and can simply be compensated with LARS; only variable or drifting orientation means the stabilization design itself is failing and must be changed.

    Source: Efron, Contact Lens Practice — stability of soft toric lens orientation and axis compensationReport a problem with this question

17. 17. A simultaneous-vision multifocal soft lens is assessed three minutes after insertion; distance vision fluctuates and the lens decenters temporally. What should be done first?

    • A.Order a steeper base curve now, because temporal decentration means a flat fit.
    • B.Convert the patient to monovision now, since multifocal optics need centration.
    • C.Let the lens settle, then reassess centration and vision before changing it.✓ Answer
    • D.Add plus to the distance power now, to clear the fluctuating distance vision.

    A freshly inserted lens is still equilibrating with the tear film and the lid, so early decentration and fluctuating vision are expected and do not yet describe the fit. Judging centration and acuity only after the lens has settled prevents unnecessary parameter changes and unnecessary redesigns.

    Source: Efron, Contact Lens Practice — fitting and assessment of multifocal soft contact lensesReport a problem with this question

18. 18. How should an over-refraction over a diagnostic contact lens be performed?

    • A.With the phoropter immediately after insertion, before the lens has settled at all.
    • B.With keratometry readings alone, since the tear lens makes refraction unreliable.
    • C.With the spectacle prescription still in place, adding the lens power to it.
    • D.With handheld trial lenses in 0.25 D steps, once the lens has settled and vision is steady.✓ Answer

    The over-refraction measures what the lens plus tear lens actually leaves uncorrected, so it is only valid once the diagnostic lens has settled and acuity has stopped changing. Handheld lenses in small steps let the patient keep both eyes open and a natural head position, which the phoropter's restricted aperture does not allow.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — over-refraction technique in diagnostic lens fittingReport a problem with this question

19. 19. A spherical GP lens is fitted on a cornea with 1.75 D of with-the-rule corneal astigmatism. The best spherical over-refraction gives 20/30, but adding -1.00 x 090 over the lens gives 20/20. What does this indicate?

    • A.Residual astigmatism, which calls for a toric lens design, not a curve change.✓ Answer
    • B.An overly steep peripheral curve system, which is corrected by widening and reblending.
    • C.A decentered optic zone, which is corrected by a smaller overall diameter.
    • D.Lens flexure on the cornea, which is corrected by a steeper base curve fit.

    The tear lens under a spherical rigid lens already neutralizes the corneal astigmatism, so any cylinder still needed over the lens is residual astigmatism arising mostly from the crystalline lens. No base curve or peripheral curve change can correct it, because it is not produced by the front corneal surface; a front-surface toric or bitoric design is required.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — residual astigmatism with rigid lensesReport a problem with this question

20. 20. A GP lens made from a UV-absorbing material gives a fluorescein pattern that is hard to see under cobalt blue light. What improves the view of the pattern?

    • A.Switching to diffuse white illumination, which displays the tear layer far more clearly.
    • B.Raising the magnification and narrowing the beam to a fine optic section.
    • C.Adding a yellow barrier filter to the observation system with the cobalt light.✓ Answer
    • D.Instilling high-molecular-weight fluorescein, which is made for rigid lenses.

    A UV-absorbing lens material soaks up part of the exciting wavelengths, so the green fluorescence emitted by the dye is weak and is washed out by reflected blue light. A yellow barrier filter in front of the observation system blocks that reflected blue while passing the green emission, which restores contrast. High-molecular-weight fluorescein is the dye used with soft lenses, not rigid ones.

    Source: Bennett & Henry, Clinical Manual of Contact Lenses — fluorescein evaluation with UV-absorbing rigid lens materialsReport a problem with this question

Practice questions based on the ABO-NCLE Contact Lens Registry Exam content domains and standard contact lens references, together with the FTC Contact Lens Rule and FDA device classification. This site is not affiliated with or endorsed by ABO-NCLE. Contact lens fitting is regulated state by state — scope of practice, supervision and licensure requirements vary, so confirm your own state's rules and work under the prescriber's direction. Verify current exam requirements with ABO-NCLE before testing. About the ABO-NCLE exams →