20 Refraction, Lensometry & Optics Practice Questions & Answers

1. 1. A spectacle prescription reads +2.00 -1.00 x 180. Written in plus-cylinder form, it is:

   • A.+1.00 +1.00 x 090✓ Answer
   • B.+1.00 +1.00 x 180
   • C.+3.00 +1.00 x 090
   • D.+2.00 +1.00 x 090

   Transposition has three steps: add the cylinder algebraically to the sphere (+2.00 plus -1.00 = +1.00), reverse the sign of the cylinder (-1.00 becomes +1.00), and rotate the axis 90 degrees while keeping it between 1 and 180 (180 becomes 090). Both forms describe exactly the same lens, so neither is more correct than the other.

   Source: Ophthalmic optics: cylinder transposition ruleReport a problem with this question

2. 2. A refraction is recorded as -3.00 -2.00 x 180. What is the spherical equivalent?

   • A.-4.00 D✓ Answer
   • B.-2.00 D
   • C.-3.00 D
   • D.-5.00 D

   The spherical equivalent is the sphere plus half the cylinder: -3.00 plus (-1.00) equals -4.00 D. Transposing the same lens to plus-cylinder form gives -5.00 +2.00 x 090, whose spherical equivalent is also -4.00 D, because the spherical equivalent is the circle of least confusion of the conoid of Sturm and does not depend on which cylinder form is written.

   Source: Ophthalmic optics: spherical equivalent (sphere plus half the cylinder)Report a problem with this question

3. 3. A patient wears a -12.00 D spectacle lens. Moving that lens closer to the eye:

   • A.Leaves the required power unchanged
   • B.Matters only for low-powered lenses
   • C.Reduces the minus power the eye needs✓ Answer
   • D.Increases the minus power the eye needs

   Effective power at the eye depends on the distance from the lens to the cornea. As a minus lens is brought closer, its effective minus power at the corneal plane rises, so less minus is needed to give the same correction. The size of this shift grows with lens power, which is why vertex distance is measured and allowed for in strong prescriptions but is negligible in weak ones.

   Source: Clinical optics: effective power and vertex distanceReport a problem with this question

4. 4. Why does a presbyopic patient need a near add?

   • A.The vitreous liquefies with age and scatters the image
   • B.The crystalline lens no longer adds enough power for near✓ Answer
   • C.The pupil constricts with age and admits less light
   • D.The cornea flattens with age and loses refractive power

   Accommodation works by the crystalline lens becoming more convex when the ciliary muscle contracts. With age the lens stiffens and the amplitude of accommodation falls, so near objects focus behind the retina; a plus add supplies the converging power the lens can no longer generate. The add is the same for both eyes and grows as the accommodative amplitude declines.

   Source: Ocular physiology: accommodation and presbyopiaReport a problem with this question

5. 5. During retinoscopy the reflex moves in the same direction as the streak. This with-motion tells the examiner to:

   • A.Move farther from the patient and re-check
   • B.Add plus power until the reflex neutralizes✓ Answer
   • C.Rotate the streak 90 degrees and re-check
   • D.Add minus power until the reflex neutralizes

   With-motion means the eye's far point lies beyond the examiner's working distance, so converging power must be added: plus lenses are added until neutrality. Against-motion means the opposite and calls for minus. At neutrality the reflex fills the pupil and no longer moves in either direction.

   Source: Retinoscopy principles: with-motion and against-motionReport a problem with this question

6. 6. Gross retinoscopy over the right eye is +2.50 D with a +1.50 D working-distance lens in place. What is the net finding?

   • A.+4.00 D
   • B.-1.00 D
   • C.+1.00 D✓ Answer
   • D.+2.50 D

   The working-distance lens compensates for the examiner sitting a finite distance from the patient, so its dioptric value must be subtracted from the gross finding: +2.50 minus +1.50 equals +1.00 D. Forgetting to subtract it is the classic error and produces a systematic over-minus in every eye measured.

   Source: Retinoscopy: working-distance lens allowanceReport a problem with this question

7. 7. On the red-green (duochrome) test the patient says the letters on the red side are clearer. The next step is to:

   • A.Stop and record a red-green colour defect
   • B.Add plus sphere until the sides match
   • C.Accept the red preference as the endpoint
   • D.Add minus sphere until the sides match✓ Answer

   Because of longitudinal chromatic aberration, long-wavelength red light focuses behind short-wavelength green light. If the red side is clearer, the eye's overall focus sits in front of the retina, so minus sphere is added until the two sides look equally clear. The test judges blur rather than hue, so it remains valid in colour-deficient patients.

   Source: Subjective refraction: duochrome (red-green) testReport a problem with this question

8. 8. A patient reads the 20/20 line with -2.25 sphere and again with -2.50 sphere. Which sphere should be recorded?

   • A.-2.50, because more minus adds contrast
   • B.-2.50, the most minus that reads 20/20
   • C.-2.25, the least minus that reads 20/20✓ Answer
   • D.-2.375, the midpoint of the two spheres

   The endpoint rule for subjective sphere is maximum plus, or least minus, that still gives the best acuity. Extra minus is focused by the patient's own accommodation rather than by the lens, so it looks acceptable in the chair but causes eyestrain and near blur once the glasses are worn.

   Source: Subjective refraction: maximum plus / least minus endpointReport a problem with this question

9. 9. When a cross cylinder is used to refine an existing cylinder, the correct order is to:

   • A.Refine the power first, then the axis
   • B.Refine the sphere first, then the axis
   • C.Adjust axis and power in one step
   • D.Refine the axis first, then the power✓ Answer

   An incorrect axis blurs both principal meridians, so any judgement of cylinder power made before the axis is right is unreliable; axis is therefore refined first and power second. When cylinder power is changed, the sphere is moved half that amount in the opposite direction so that the spherical equivalent, and therefore the overall blur, stays constant.

   Source: Subjective refraction: cross-cylinder refinement sequenceReport a problem with this question

10. 10. Binocular balance by prism dissociation is appropriate only when:

    • A.The two eyes have different powers
    • B.Best corrected acuity is equal in both eyes✓ Answer
    • C.The patient has an intermittent exotropia
    • D.The refraction was done under cycloplegia

    Balancing works by comparing the blur seen by the two eyes at the same time, which is only meaningful if each eye can reach the same best corrected acuity. If one eye sees worse for organic reasons, the images can never be matched and the technique gives a false result, so the endpoint of maximum plus is applied to each eye separately instead.

    Source: Subjective refraction: binocular balance by prism dissociationReport a problem with this question

11. 11. A 6-year-old with esotropia is scheduled for cycloplegic refraction. The cycloplegic agent is used because it:

    • A.Reduces glare from an unstable tear film
    • B.Steadies fixation during the retinoscopy
    • C.Relaxes accommodation and unmasks hyperopia✓ Answer
    • D.Dilates the pupil and brightens the reflex

    Children have a large accommodative amplitude and can hold part of their hyperopia hidden throughout an ordinary refraction. Paralysing the ciliary muscle removes that tone so the full hyperopic error appears, which matters because accommodative esotropia is driven by the accommodation used to overcome uncorrected hyperopia.

    Source: Refraction in children: cycloplegic refractionReport a problem with this question

12. 12. Before neutralizing a patient's spectacles, the first step at the lensmeter is to:

    • A.Seat the lens against the stop
    • B.Turn the power drum to plano
    • C.Focus the eyepiece on the reticle✓ Answer
    • D.Mark the optical center with ink

    The eyepiece is turned to full plus and then back until the reticle crosshairs are crisp, which cancels the examiner's own accommodation. If that step is skipped, the observer accommodates to focus the mires and every sphere reading is shifted in the same direction, so the error is systematic rather than random.

    Source: Lensometry procedure: focusing the eyepieceReport a problem with this question

13. 13. To read the near add of a patient's bifocal at the lensmeter, the technician should:

    • A.Average the distance and near readings
    • B.Rest the front surface on the lens stop✓ Answer
    • C.Rest the back surface on the lens stop
    • D.Record the near reading as the add

    The distance prescription is a back vertex measurement, but the add is defined as a front vertex measurement, so the spectacle is turned around with the front surface against the stop. The distance portion is read in that position and the segment is read next; the add is the difference between the two.

    Source: Lensometry: front vertex measurement of the near addReport a problem with this question

14. 14. At the lensmeter one set of mires focuses at one drum setting and the second set at another. The cylinder power is:

    • A.The larger of the two drum readings
    • B.The sum of the two drum readings
    • C.The difference between the readings✓ Answer
    • D.The average of the two drum readings

    A spherocylindrical lens has two principal meridians with different powers, and each brings one set of mires into focus. The first reading is taken as the sphere and the difference between the two readings is the cylinder, with its sign set by which meridian was read first; the axis is read from the scale where those lines are sharp.

    Source: Lensometry: reading sphere, cylinder and axisReport a problem with this question

15. 15. A lens is +4.00 D and its optical center sits 5 mm from the patient's line of sight. By Prentice's rule (prism diopters equal decentration in centimeters times power), the induced prism is:

    • A.2.0 prism diopters✓ Answer
    • B.0.2 prism diopters
    • C.20 prism diopters
    • D.5.0 prism diopters

    Decentration must be expressed in centimeters, so 5 mm becomes 0.5 cm and 0.5 times 4.00 gives 2.0 prism diopters. Leaving the decentration in millimeters is the standard error and inflates the answer tenfold. At the lensmeter such a lens shows its mires displaced from the reticle center, and the prism is named by the direction of its base.

    Source: Clinical optics: Prentice's ruleReport a problem with this question

16. 16. While neutralizing a patient's glasses the mires never come to a single sharp focus and drift steadily as the lens is moved downward. This behaviour indicates:

    • A.A lens warped by heat during adjustment
    • B.A lens with ground-in vertical prism
    • C.A lined bifocal with a visible segment
    • D.A progressive lens with a blending zone✓ Answer

    In a progressive lens the power changes continuously down the corridor, so there is no single plane of constant power to neutralize and the mires blur and shift as the lens moves. A lined multifocal instead gives two discrete, separately readable zones; for a progressive, the engravings and the marked distance reference circle are used to locate the reading position.

    Source: Lensometry: progressive addition lensesReport a problem with this question

17. 17. Keratometry readings look normal, but the patient's vision fluctuates and the cornea appears irregular at the slit lamp. The most useful next measurement is:

    • A.Topography of the full corneal surface✓ Answer
    • B.Ultrasound biometry to measure axial length
    • C.Repeated keratometry of the other eye
    • D.Lensometry of the current spectacles

    A keratometer samples only a few points on the central anterior cornea, so a distorted mid-peripheral or inferior cornea can hide behind readings that look ordinary. Topography maps the whole anterior surface and reveals the localized steepening and irregularity that keratometry cannot see.

    Source: Keratometry: central anterior corneal sampling; corneal topographyReport a problem with this question

18. 18. The keratometer mires look broken and distorted in a patient who reports burning and blurred vision. Before reporting corneal irregularity, the technician should:

    • A.Record the distorted readings without comment
    • B.Reduce the room lights and repeat the readings
    • C.Have the patient blink and repeat the readings✓ Answer
    • D.Use the other eye's values for this eye

    Keratometry reflects mires off the tear film rather than off the epithelium itself, so a broken-up or dry tear film scatters the reflection and mimics true corneal irregularity. Re-establishing a smooth film with a blink, or with lubrication, and repeating separates an ocular surface problem from genuine corneal disease.

    Source: Keratometry: tear film and mire qualityReport a problem with this question

19. 19. Corneal power in diopters equals 337.5 divided by the radius of curvature in millimeters. A radius of 7.50 mm corresponds to:

    • A.42.00 D
    • B.45.00 D✓ Answer
    • C.47.50 D
    • D.44.00 D

    Dividing 337.5 by 7.50 gives 45.00 D. The relationship is a reciprocal one, so a shorter radius means a steeper, more powerful cornea and a longer radius a flatter, weaker one; the same expression rearranged converts a dioptric reading back into the radius needed when a base curve is chosen for a contact lens.

    Source: Keratometry: standard keratometric conversion between radius and dioptresReport a problem with this question

20. 20. Keratometry is recorded as 42.00 @ 180 and 44.00 @ 090. This cornea shows:

    • A.Against-the-rule astigmatism of 2.00 D
    • B.With-the-rule astigmatism of 2.00 D✓ Answer
    • C.With-the-rule astigmatism of 4.00 D
    • D.Oblique corneal astigmatism of 2.00 D

    Corneal astigmatism is the steep reading minus the flat reading, here 44.00 minus 42.00, and the steep meridian lying near 090 defines with-the-rule; a steep meridian near 180 would be against-the-rule and anything well away from those two is oblique. Corneal astigmatism is not the same as the patient's total refractive astigmatism, because the crystalline lens contributes its own component.

    Source: Keratometry: recording principal meridians and corneal astigmatismReport a problem with this question

Practice questions based on the IJCAHPO core certification content areas for the Certified Ophthalmic Assistant and on standard ophthalmic clinical practice references. COA, COT, and COMT are marks of the International Joint Commission on Allied Health Personnel in Ophthalmology; this site is not affiliated with or endorsed by IJCAHPO. Ophthalmic assistants work under the delegation of a supervising ophthalmologist and permitted tasks vary by state and by practice — confirm your own scope, and check the current exam requirements and content outline with IJCAHPO before testing. About IJCAHPO certification →