20 Optics & Refractive Errors Practice Questions & Answers

1. 1. Which statement describes myopia and the way a contact lens corrects it?

   • A.Light focuses behind the retina and a plus lens converges it back onto the retina
   • B.Light forms two separate focal lines and a cylindrical lens combines them
   • C.Light focuses in front of the retina and a minus lens diverges it onto the retina✓ Answer
   • D.Light focuses in front of the retina and a plus lens converges it onto the retina

   In myopia the refracting power of the eye is too strong for its axial length, so parallel light comes to focus in front of the retina. A minus-powered lens diverges the incoming light and moves the focal point back onto the retina; adding plus power would push the focus further forward and increase the blur.

   Source: Bennett & Weissman, Clinical Contact Lens Practice — optics of ametropia and contact lens correctionReport a problem with this question

2. 2. A 26-year-old reports clear distance vision but headaches and blurring after an hour of reading. The distance refraction is +2.25 DS in each eye. Which explanation fits this finding?

   • A.The eye is presbyopic; the crystalline lens has lost most of its accommodative range
   • B.The eye is astigmatic; light forms two focal lines that accommodation cannot merge
   • C.The eye is myopic; light focuses in front of the retina and near tasks stay easy
   • D.The eye is hyperopic; light focuses behind the retina and accommodation clears it✓ Answer

   In hyperopia the refracting power of the eye is too weak for its axial length and light comes to focus behind the retina. A 26-year-old still has ample accommodation and can clear distance vision, but the additional accommodation demanded at near produces asthenopia and blur after sustained reading, which plus correction relieves. Presbyopia would not be expected at this age.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — hyperopia and accommodative demandReport a problem with this question

3. 3. A spectacle refraction is written -3.25 +1.25 x 180. How is this astigmatism classified?

   • A.Mixed astigmatism, because one focal line falls in front of the retina and one behind
   • B.Against-the-rule, because the steeper corneal meridian lies close to horizontal✓ Answer
   • C.With-the-rule, because the steeper corneal meridian lies close to vertical
   • D.Oblique, because both principal meridians lie between 31 and 59 degrees

   Transposed to minus cylinder the prescription is -2.00 -1.25 x 090. A minus-cylinder axis near 090 means the correcting minus cylinder acts in the horizontal meridian, so the steeper meridian of the eye is horizontal, which is the definition of against-the-rule astigmatism. Reading a plus-cylinder axis as though it were a minus-cylinder axis is the usual error on this type of item.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — classification of regular astigmatism and transpositionReport a problem with this question

4. 4. Before designing a rigid lens, a fitter transposes the refraction -1.75 +2.25 x 045 into minus-cylinder form. What is the result?

   • A.+0.50 -2.25 x 135✓ Answer
   • B.+0.50 -2.25 x 045
   • C.+0.50 +2.25 x 135
   • D.-0.50 -2.25 x 135

   Transposition has three steps: add the cylinder power to the sphere (-1.75 + 2.25 = +0.50), reverse the sign of the cylinder (-2.25) and rotate the axis by 90 degrees (045 becomes 135). Rigid lens work is done in minus cylinder because in that form the sphere power corresponds to the flattest corneal meridian, which is the meridian a rigid lens is usually fitted on.

   Source: CLSA Contact Lens Manual, Volume 1 — transposition of sphero-cylindrical prescriptionsReport a problem with this question

5. 5. Which statement best describes presbyopia and its usual optical management?

   • A.A gradual increase in axial length, managed by adding minus power for distance
   • B.A gradual increase in corneal toricity, managed by adding cylinder power at near
   • C.A gradual decrease in tear volume, managed by adding a rewetting agent at near
   • D.A gradual loss of accommodative amplitude, managed by adding plus power for near✓ Answer

   Presbyopia is the age-related loss of accommodative amplitude caused by reduced flexibility of the crystalline lens and its capsule, so the near point recedes beyond a comfortable working distance. It is managed optically by supplying plus power for near, whether as the add of a multifocal contact lens or as the near correction of a monovision pair.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — presbyopia and near additionReport a problem with this question

6. 6. A spectacle prescription of -8.00 DS is measured at a 12 mm vertex distance. Compared with that power, what power will the contact lens need?

   • A.Unchanged at -8.00 D, because compensation starts above 10.00 D
   • B.Weaker in minus power, closer to about -7.25 D at the cornea✓ Answer
   • C.Stronger in minus power, closer to about -8.75 D at the cornea
   • D.Unchanged at -8.00 D, because vertex affects only plus lenses

   Vertex compensation is applied once the spectacle sphere reaches about 4.00 D, and it applies to minus and plus lenses alike. Moving the correction closer to the eye shortens the distance from lens to far point, so a minus lens needs less minus at the corneal plane; Fc = Fs / (1 - d x Fs) with d = 0.012 m gives about -7.30 D, closest to -7.25 D.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — vertex distance and effective powerReport a problem with this question

7. 7. A spectacle prescription of +9.00 DS is measured at a 12 mm vertex distance. Using Fc = Fs / (1 - d x Fs), with d expressed in metres, the contact lens power is closest to which value?

   • A.+8.00 D
   • B.+11.25 D
   • C.+9.00 D
   • D.+10.00 D✓ Answer

   Substituting the values gives Fc = +9.00 / (1 - 0.012 x 9.00) = 9.00 / 0.892 = +10.09 D, closest to +10.00 D. Moving a plus lens closer to the eye requires more plus power; obtaining about +8.12 D instead means the sign in the denominator was reversed.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — vertex distance conversion for high powersReport a problem with this question

8. 8. A rigid gas permeable lens with a base curve of 43.50 D is placed on a cornea whose flattest keratometry reading is 42.75 D. What is the power of the tear lens formed beneath it?

   • A.-0.75 D, since a lens steeper than K forms a minus tear lens
   • B.Plano, since the tear layer and the cornea share the same index
   • C.+0.75 D, since a lens steeper than K forms a plus tear lens✓ Answer
   • D.+1.50 D, since the tear lens equals twice the curvature difference

   Tear lens power equals the base curve in dioptres minus the flattest keratometry reading: 43.50 - 42.75 = +0.75 D. A base curve steeper than flat K leaves the tear layer thicker centrally than at its edge, which is a plus-powered lens; this is precisely why steepening a rigid lens requires minus power to be added to keep the total correction the same.

   Source: AAO Basic and Clinical Science Course, Clinical Optics — the tear (lacrimal) lens in rigid contact lens fittingReport a problem with this question

9. 9. A rigid lens of -2.00 D on a base curve of 42.00 D gives the patient exactly the correction needed. The fitter now orders the same design on a base curve of 43.00 D. What lens power keeps the total correction the same?

   • A.-1.00 D, applying plus power for the steeper base curve
   • B.-4.00 D, applying minus twice for the 1.00 D of steepening
   • C.-3.00 D, applying minus power for the steeper base curve✓ Answer
   • D.-2.00 D, because lens power never changes with base curve

   The new base curve is 1.00 D steeper than the original, which introduces a +1.00 D tear lens under the lens. To keep the power reaching the eye unchanged, an equal amount of minus must be added to the lens itself, the rule remembered as SAM, Steeper Add Minus: -2.00 + (-1.00) = -3.00 D.

   Source: CLSA Contact Lens Manual, Volume 1 — base curve and power relationship (SAM/FAP)Report a problem with this question

10. 10. A diagnostic rigid lens of -1.50 D on a base curve of 42.50 D gives a spherical over-refraction of -0.50 D. The fitter decides to order the lens on a base curve of 43.00 D. What power should be ordered?

    • A.-1.50 D
    • B.-3.00 D
    • C.-2.50 D✓ Answer
    • D.-2.00 D

    Work the problem in order. First combine the over-refraction with the diagnostic lens power: -1.50 + (-0.50) = -2.00 D, the power that actually corrects this eye on the trial base curve. Then apply SAM for the 0.50 D of steepening: -2.00 + (-0.50) = -2.50 D. Reversing those two steps yields -1.50 D, a common error that is offered as a distractor.

    Source: CLSA Contact Lens Manual, Volume 1 — over-refraction combined with a base curve change (SAM/FAP)Report a problem with this question

11. 11. A spectacle refraction is -2.75 -1.00 x 175 and a spherical soft lens is to be ordered. What is the spherical equivalent of this prescription?

    • A.-2.75 DS
    • B.-3.25 DS✓ Answer
    • C.-3.75 DS
    • D.-2.25 DS

    The spherical equivalent is the sphere plus half the cylinder: -2.75 + (-1.00 / 2) = -3.25 D. It places the correction at the circle of least confusion within the conoid of Sturm, which is why it is the value used when a spherical soft lens must be chosen for a patient with a low amount of cylinder.

    Source: AAO Basic and Clinical Science Course, Clinical Optics — spherical equivalent and the conoid of SturmReport a problem with this question

12. 12. A patient's entire refractive cylinder of -1.50 x 180 arises from the front surface of the cornea. What happens to that cylinder with a spherical rigid lens compared with a spherical soft lens?

    • A.Both lens types neutralise the cylinder because each replaces the corneal surface
    • B.The rigid lens leaves the cylinder uncorrected; the soft lens neutralises it
    • C.Neither lens type neutralises the cylinder because corneal cylinder is lenticular
    • D.The rigid lens neutralises the cylinder; the soft lens leaves it uncorrected✓ Answer

    A rigid lens keeps its own spherical front surface and the tear layer fills in the corneal toricity, so cylinder arising on the front of the cornea is effectively neutralised. A soft lens drapes onto the cornea and takes up its toricity, reproducing the same cylinder on its own front surface, so the cylinder remains uncorrected. That is why one patient may need a toric soft lens but only a spherical rigid lens.

    Source: Mandell, Contact Lens Practice — residual astigmatism with rigid and soft lensesReport a problem with this question

13. 13. Keratometry reads 42.00 D @ 180 and 43.00 D @ 090. The spectacle refraction in minus-cylinder form is -2.00 -1.50 x 180. How much of the astigmatism is lenticular?

    • A.Plano, because all of the cylinder lies on the cornea
    • B.-0.50 x 180, the total cylinder minus the corneal cylinder✓ Answer
    • C.-1.00 x 180, the cylinder value read by the keratometer
    • D.-2.50 x 180, the total cylinder added to the corneal cylinder

    The keratometer shows 1.00 D of corneal toricity with the steep meridian at 090, which is written -1.00 x 180 in minus cylinder. Lenticular, or internal, astigmatism is the total refractive cylinder minus the corneal cylinder: -1.50 - (-1.00) = -0.50 x 180. Only the corneal component can be neutralised by the tear lens of a rigid lens.

    Source: Bennett & Weissman, Clinical Contact Lens Practice — corneal versus lenticular (internal) astigmatismReport a problem with this question

14. 14. The spectacle refraction is -3.00 -1.75 x 180 and keratometry reads 43.00 D @ 180 / 44.25 D @ 090. A spherical rigid lens is fitted on the flattest meridian. What residual astigmatism should be anticipated?

    • A.-0.50 x 180✓ Answer
    • B.-1.25 x 180
    • C.-1.75 x 180
    • D.-3.00 x 180

    Keratometry shows 1.25 D of corneal astigmatism with the steep meridian at 090, that is -1.25 x 180 in minus cylinder. Anticipated residual astigmatism is the total refractive cylinder minus the corneal cylinder, because the tear lens beneath a spherical rigid lens neutralises the corneal component: -1.75 - (-1.25) = -0.50 x 180.

    Source: Mandell, Contact Lens Practice — calculation of anticipated residual astigmatismReport a problem with this question

15. 15. A patient is -1.00 D in the right eye and -6.00 D in the left. Optically, why are contact lenses usually preferred to spectacles for this patient?

    • A.They raise the accommodative demand equally in the two eyes at every working distance
    • B.They reduce corneal astigmatism in the more myopic eye by masking it with tears
    • C.They largely equalise retinal image size and avoid prismatic imbalance in downgaze✓ Answer
    • D.They add base-down prism before the more myopic eye to balance the retinal images

    Because a contact lens sits at the corneal plane rather than millimetres in front of it, it produces far less difference in retinal image size between two very unequal eyes, so aniseikonia is minimised. The lens also moves with the eye, so no vertical prismatic imbalance is induced when the patient looks away from the optical centres, which is the usual complaint of a high anisometrope in spectacles.

    Source: AAO Basic and Clinical Science Course, Clinical Optics — anisometropia, aniseikonia and contact lens correctionReport a problem with this question

16. 16. Why must a toric soft contact lens include a stabilisation feature such as prism ballast or thin zones?

    • A.It raises oxygen transmissibility across the zone that carries the cylinder power
    • B.It lowers the sagittal depth so that the lens moves more freely on each blink
    • C.It centres the optic zone over the pupil so that flare is avoided in dim light
    • D.It holds the cylinder axis at the prescribed orientation as the lens settles✓ Answer

    A soft lens has no intrinsic reason to adopt any particular orientation on the eye, yet a cylinder only corrects when its axis lies in the prescribed meridian. Prism ballast, thin superior and inferior zones or truncation use lid forces and lens thickness distribution to bring the lens to a repeatable orientation and hold it there through the blink cycle.

    Source: Bennett & Weissman, Clinical Contact Lens Practice — soft toric lens stabilisationReport a problem with this question

17. 17. A soft toric lens made to -2.25 -1.25 x 020 sits on the eye with its scribe mark rotated 10 degrees counterclockwise as the fitter views it. Vision is otherwise good. Applying LARS, what axis should be ordered?

    • A.030, adding the rotation to the ordered axis
    • B.020, leaving the ordered axis exactly as it is
    • C.040, adding double the observed rotation
    • D.010, subtracting the rotation from the ordered axis✓ Answer

    LARS stands for Left Add, Right Subtract, and the direction is judged from the practitioner's view of the lens marking. A counterclockwise shift is a rotation to the fitter's left, so the observed 10 degrees is added to the prescribed axis: 020 + 10 = 030. Ordering the uncompensated axis leaves roughly a third of the cylinder effect unused.

    Source: CLSA Contact Lens Manual, Volume 1 — LARS rule for toric lens rotationReport a problem with this question

18. 18. A simultaneous-vision multifocal contact lens presents distance and near images to the retina at the same moment. What consequence should the wearer be counselled to expect?

    • A.Contrast is reduced and haloes may appear, because a blurred image is always present✓ Answer
    • B.The cylinder axis rotates on blink, because the near zone weights the lower edge
    • C.Stereopsis is lost completely, because each eye is corrected for a different distance
    • D.Near power works only in downgaze, because the lens shifts up on the lower lid

    In a simultaneous-vision design both the distance and the near portions of the optic zone lie within the pupil at the same time, so the retina always receives a defocused image superimposed on the focused one. That superimposed blur lowers contrast and can produce haloes or ghosting, most noticeably at night when the pupil is large.

    Source: Bennett & Weissman, Clinical Contact Lens Practice — simultaneous vision multifocal designsReport a problem with this question

19. 19. Monovision corrects the dominant eye for distance and the fellow eye for near. What is the principal optical trade-off of this approach?

    • A.Accommodative amplitude is further reduced, most noticeably in the dominant eye
    • B.Corneal astigmatism is induced, most noticeably in the eye corrected for near
    • C.Binocular summation and stereoacuity are reduced, most noticeably in dim light✓ Answer
    • D.Peripheral visual field is restricted, most noticeably in the non-dominant eye

    Monovision leaves the two eyes with unequal retinal image quality at any single viewing distance, so the visual system cannot fuse two equally sharp images. Binocular summation and stereoacuity therefore fall, and the loss is most apparent in dim light and at intermediate distances such as night driving. Monovision does not induce corneal cylinder or narrow the visual field.

    Source: Bennett & Weissman, Clinical Contact Lens Practice — monovision correction of presbyopiaReport a problem with this question

20. 20. Compared with a spectacle lens of equivalent corrected power, what does a contact lens produce for a high myope?

    • A.An unchanged retinal image but a narrower field, because the optic zone is small
    • B.A larger retinal image and a wider field of view, because the lens sits on the cornea✓ Answer
    • C.A larger retinal image but a narrower field of view, because of the lens edge design
    • D.A smaller retinal image and a narrower field of view, because the lens sits on the cornea

    Spectacle magnification depends on how far the correcting lens sits from the eye's entrance pupil. A minus lens held about 12 mm in front of the eye minifies the retinal image, so moving the same correction to the corneal plane removes most of that minification and the myope perceives a larger image. The contact lens also rotates with the eye and has no frame, so the usable field is wider and there is no ring scotoma at the lens edge.

    Source: AAO Basic and Clinical Science Course, Clinical Optics — spectacle magnification and field of view with contact lensesReport 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 →