21 Instruments & Verification Practice Questions & Answers

1. 1. A lensmeter is used to verify a finished single-vision lens. Which quantity does a standard lensmeter read when the back surface of the lens rests against the lens stop?

   • A.The true equivalent power of the lens
   • B.The refractive index of the lens material
   • C.The front surface curve of the lens
   • D.The back vertex power of the lens✓ Answer

   A lensmeter measures back vertex power, the reciprocal of the distance from the back surface of the lens to its focal point, because the back surface is the reference plane sitting on the lens stop. Equivalent power and surface curves are different quantities and are not what the instrument reports, and no lensmeter reads refractive index.

   Source: American Board of Opticianry exam content outline, instrumentation; System for Ophthalmic DispensingReport a problem with this question

2. 2. Before neutralizing any lens in a lensmeter, the operator must first focus the eyepiece. What is the reason this step comes first?

   • A.It levels the spectacle table so both eyewires sit in the same plane
   • B.It calibrates the axis wheel so the axis scale aligns with the 180 meridian
   • C.It cancels the operator's own accommodation so the reticle is sharp at the plano setting and readings are not biased✓ Answer
   • D.It sets the prism compensator to zero so no unwanted prism is introduced

   An unfocused eyepiece lets the operator accommodate to bring the target clear, which shifts every power reading in the plus or minus direction by a constant amount. Focusing the eyepiece first, with the reticle sharp while the power drum reads plano, removes that bias; if the mires cannot be made sharp at plano the instrument needs recalibration.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; System for Ophthalmic DispensingReport a problem with this question

3. 3. While reading a minus-cylinder lens in a lensmeter, the single thin lines come into sharp focus at -1.75 D and the wide triple lines come into focus at -3.25 D with the axis wheel at 070. What is the lens power?

   • A.-1.75 -1.50 x 160
   • B.-1.75 -3.25 x 070
   • C.-3.25 -1.75 x 070
   • D.-1.75 -1.50 x 070✓ Answer

   The sphere is the first meridian focused, and the cylinder is the difference travelled on the power drum between the two focus points, so -3.25 minus -1.75 gives -1.50 D of cylinder. Reading the second drum number as the cylinder is the classic error, and the axis is read directly from the axis wheel when the sphere lines are aligned.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; System for Ophthalmic DispensingReport a problem with this question

4. 4. An optician places a lens in the lensmeter and finds that the thin lines and the wide lines come into sharp focus at the same drum setting no matter how the axis wheel is turned. What does this indicate about the lens?

   • A.The lens has cylinder power but the axis is at 180
   • B.The lens contains prism at its optical center
   • C.The lens is spherical, with no cylinder power✓ Answer
   • D.The lens is a progressive design read at the fitting cross

   Cylinder power exists only when two principal meridians have different powers, which makes the two sets of target lines focus at two different drum settings. When both sets focus together at one setting and rotation of the axis wheel changes nothing, every meridian has the same power, so the lens is spherical.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; System for Ophthalmic DispensingReport a problem with this question

5. 5. A patient reports eyestrain with new glasses. The optician marks the patient's monocular PD and OC height on each lens with a layout chart, places the right dot on the lensmeter stop, and sees the target displaced toward the patient's nose. How should this finding be recorded?

   • A.Vertical prism, base up at the patient's line of sight
   • B.Base-in prism at the patient's line of sight✓ Answer
   • C.No prism, because the target can be recentered by sliding the lens
   • D.Base-out prism at the patient's line of sight

   Base direction follows the direction in which the target is displaced, so a target shifted toward the nose is base-in prism. The measurement must be taken at the marked monocular PD and OC height because that is where the patient actually looks; sliding the lens to recenter the target simply hides the prism the wearer is experiencing.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; System for Ophthalmic DispensingReport a problem with this question

6. 6. An optician needs to verify the add power of a flat-top bifocal whose segment is on the front surface. What must be done differently from reading the distance power?

   • A.Read the add only at the geometric center of the lens with the back surface on the stop
   • B.Leave the spectacles as they are and read the segment with the axis wheel set to 090
   • C.Read the add from the etched markings near the temporal edge of the lens
   • D.Turn the spectacles around so the front surface rests on the lens stop and read the distance and near zones from the front✓ Answer

   For a front-surface segment the add is defined as the difference between the front vertex power of the distance portion and the front vertex power of the near portion, so the frame is reversed with the temples toward the operator and both readings are taken from the front. Reading a front segment from the back introduces a thickness-related error that grows with the strength of the add, and etched markings belong to progressive lenses, not lined bifocals.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; ANSI Z80.1 ophthalmic prescription lens standard, add power measurementReport a problem with this question

7. 7. Two opticians measure the same lined bifocal with a +3.00 D nominal add: one reads it with the back surface against the lens stop and the other reverses the frame and reads from the front. Why do the two readings differ?

   • A.The distance and near portions have different thicknesses and vertex positions, so back vertex readings do not subtract cleanly while front vertex readings do✓ Answer
   • B.The lens stop applies pressure that flexes the segment when read from the back
   • C.The lensmeter is more sensitive to plus power than to minus power
   • D.Reversing the frame changes the axis reading, which alters the add

   The near portion of a front-surface bifocal is thicker than the distance portion, so the back vertex powers of the two zones are referenced to different effective positions and their difference is not the true add. Measuring both zones from the front puts them on a common reference surface, which is why the reversed-lens method is the specified way to verify the add, and the discrepancy grows as the add gets stronger.

   Source: ANSI Z80.1 ophthalmic prescription lens standard, add power measurement; System for Ophthalmic DispensingReport a problem with this question

8. 8. An optician receives a general-purpose progressive lens for verification. Which reference point on the lens is used to confirm the add power?

   • A.The prism reference dot below the engravings
   • B.The geometric center of the finished lens
   • C.The permanent engraving that carries the add value, on the temporal side✓ Answer
   • D.The fitting cross of the re-inked layout markings

   A progressive has no abrupt near zone to neutralize cleanly, so the add is confirmed from the permanent engraving that carries the add value rather than by hunting for a reading in the corridor. The fitting cross is a fitting reference used for height placement, and the prism reference dot is where prescribed prism is checked, so neither gives the add.

   Source: American Board of Opticianry exam content outline, use of lens power measuring devices; ANSI Z80.1 ophthalmic prescription lens standard, progressive addition lens markingsReport a problem with this question

9. 9. Prescribed prism is ordered in a general-purpose progressive lens. At which marking should the optician place the lens on the lensmeter stop to verify that prism?

   • A.The fitting cross
   • B.The prism reference point marked between and below the permanent engravings✓ Answer
   • C.The circled add engraving on the temporal side
   • D.The near reference circle of the layout markings

   Prescribed prism in a progressive is specified at the prism reference point, so that is the only marking where the ordered prism value can legitimately be confirmed. The fitting cross is a placement reference in front of the pupil, the circled engraving carries the add value, and the near circle is used to check the near zone, so none of them is the prism reference.

   Source: ANSI Z80.1 ophthalmic prescription lens standard, progressive addition lens reference points; System for Ophthalmic DispensingReport a problem with this question

10. 10. During hand neutralization with trial lenses, an optician moves an unknown lens side to side in front of a distant target and the target appears to move in the same direction as the lens. What does this indicate?

    • A.The lens is plano and no trial lens is needed
    • B.The lens is minus, showing 'with' motion, and is neutralized by adding plus trial lenses✓ Answer
    • C.The lens is plus, showing 'against' motion, and is neutralized by adding minus trial lenses
    • D.The lens is plus, showing 'with' motion, and is neutralized by adding plus trial lenses

    A minus lens forms a virtual, erect image, so the target appears to travel with the lens, which is 'with' motion; a plus lens inverts the image beyond its focal point and produces 'against' motion. Neutralization consists of stacking trial lenses of opposite sign until all apparent motion stops, so a 'with' lens is neutralized with plus trial power.

    Source: American Board of Opticianry exam content outline, neutralization of lenses; System for Ophthalmic DispensingReport a problem with this question

11. 11. A lens clock is placed on the front surface of a lens and rotated slowly through 180 degrees. The reading changes from +6.00 to +8.00 as it turns. What does this tell the optician about that surface?

    • A.The lens material has an index higher than the clock's calibration index
    • B.The lens has 2.00 D of unwanted prism at that point
    • C.The surface is spherical with a base curve of +7.00
    • D.The surface is toric, and +6.00 and +8.00 are its two principal meridians✓ Answer

    A spherical surface has the same sag in every direction, so a lens clock rotated on it gives a constant reading; a changing reading means the sagittal depth differs by meridian, which defines a toric surface. The highest and lowest readings obtained during the rotation are the two principal meridians of that surface.

    Source: American Board of Opticianry exam content outline, use of lens measurement devices; System for Ophthalmic DispensingReport a problem with this question

12. 12. A lens clock calibrated for an index of 1.53 reads +4.00 on the front surface of a lens made of material with an index of 1.66. What is the true surface power, and why does it differ from the reading?

    • A.About +3.21 D, because a higher index always yields a flatter effective surface power
    • B.Exactly +4.00 D, because the lens clock measures sagittal depth and index does not matter
    • C.About +4.98 D, because surface power scales with the actual index minus one divided by 0.53✓ Answer
    • D.About +6.64 D, because the reading is multiplied by the actual index

    The clock senses sagittal depth and converts it to power using a fixed assumed index of 1.53, so on any other material the dial value is only a nominal curve. Correcting with true power equal to the reading times the actual index minus one divided by 0.53 gives 4.00 times 0.66 divided by 0.53, about +4.98 D, which shows that a higher-index lens carries more true surface power than the dial indicates.

    Source: American Board of Opticianry exam content outline, use of lens measurement devices; System for Ophthalmic DispensingReport a problem with this question

13. 13. Using a lens clock, an optician reads +6.00 on the front surface and -8.50 on the back surface of a spherical lens made of a material whose index matches the clock's calibration. Using the approximate lens formula, what is the lens power, and what is the main limitation of this estimate?

    • A.+2.50 D, but the sign must be reversed for a minus lens
    • B.-2.50 D, but the formula ignores lens thickness, so it only approximates the measured back vertex power✓ Answer
    • C.-2.50 D, and it is exact because both surfaces were read on the same instrument
    • D.-14.50 D, because the two surface powers are added as absolute values

    The approximate or nominal lens formula adds the front and back surface powers with their signs, giving +6.00 plus -8.50, which equals -2.50 D. It is a thin-lens approximation that ignores center thickness, so it estimates rather than replaces the back vertex power a lensmeter reports, and the gap widens in thicker plus lenses.

    Source: American Board of Opticianry exam content outline, ophthalmic formulas; System for Ophthalmic DispensingReport a problem with this question

14. 14. An optician measures a patient's PD with a corneal reflex pupilometer and gets a smaller total than the value obtained a moment earlier for distance. What is the most likely explanation?

    • A.The target distance setting was changed to a near working distance, so convergence reduced the measured PD✓ Answer
    • B.The instrument was held too far from the patient's face, which enlarges the reflexes
    • C.The corneal reflex method cannot measure a binocular PD at all
    • D.The patient's monocular PDs are unequal, which always lowers the binocular total

    A corneal reflex pupilometer has a selectable target distance, and setting it to a near working distance makes the eyes converge, which brings the visual axes closer together and yields a smaller near PD. Unequal monocular values redistribute the total rather than shrink it, and the instrument measures both monocular and binocular PD normally.

    Source: American Board of Opticianry exam content outline, dispensing instrumentation; System for Ophthalmic DispensingReport a problem with this question

15. 15. An optician is deciding between a corneal reflex pupilometer and a millimeter PD rule for a patient being fitted with progressive lenses. Which statement best describes the practical difference?

    • A.The PD rule is preferred for progressives because it measures from the corneal reflex directly
    • B.Both instruments give only a binocular total, so monocular values must be estimated by halving
    • C.The pupilometer measures vertex distance as well, which the rule cannot do
    • D.The pupilometer isolates each eye and reads from the corneal reflex, giving monocular values with less parallax and fixation error than a hand-held rule✓ Answer

    Progressive and prism work require accurate monocular PDs because each corridor must sit in front of its own pupil, and a pupilometer isolates each eye against a controlled fixation target and reads from the corneal reflex, avoiding the parallax and head-turn errors of a hand-held rule. Halving a binocular total assumes facial symmetry that many patients do not have, and vertex distance is measured with a distometer, not a pupilometer.

    Source: American Board of Opticianry exam content outline, dispensing instrumentation; System for Ophthalmic DispensingReport a problem with this question

16. 16. Which instrument-to-measurement pairing is correct for final inspection of a finished job?

    • A.A thickness caliper measures the effective diameter of the frame's eyewire
    • B.A circumference gauge measures the center thickness of the lens
    • C.A lens clock measures the back vertex power of the finished lens
    • D.A distometer measures the distance from the back surface of the lens to the front of the cornea✓ Answer

    The distometer is built specifically to read vertex distance, the separation between the back lens surface and the front of the cornea, which is what determines whether a strong prescription needs compensation. A circumference gauge reads edge length for fitting the lens to the eyewire, a caliper reads thickness, and a lens clock reads surface curve rather than lens power.

    Source: American Board of Opticianry exam content outline, dispensing instrumentation and lens measurement devicesReport a problem with this question

17. 17. An optician verifies a job in which the stronger vertical-meridian power is -5.00 D. The optical center of that lens is dotted, and with the spectacle table untouched the same height on the other lens is dotted; the second lens's own optical center is then found 3 mm below that dot. Using Prentice's rule with the other lens's vertical power of -2.00 D, what vertical prismatic effect exists at that height?

    • A.0.6 prism diopters✓ Answer
    • B.0.3 prism diopters
    • C.6.0 prism diopters
    • D.1.5 prism diopters

    Prentice's rule states that prism equals the decentration in centimeters multiplied by the power in diopters, so 0.3 cm times 2.00 D gives 0.6 prism diopters. The unit trap is the decentration: 3 mm must be converted to 0.3 cm before multiplying, and only the power in the meridian of interest, here the vertical, is used.

    Source: American Board of Opticianry exam content outline, ophthalmic formulas; Prentice's ruleReport a problem with this question

18. 18. An optician performs the standard final inspection of a finished pair. Which sequence best reflects correct verification practice?

    • A.Measure frame alignment and coatings first, then accept the powers because the lab invoice lists them
    • B.Read the doctor's written prescription first, then neutralize the right lens and the left lens without moving the spectacle table, spot the optical centers, and compare the results with the prescription and the lab order✓ Answer
    • C.Read the lab order only, neutralize the left lens first, and reposition the table between lenses so each target centers
    • D.Spot the optical centers first, then focus the eyepiece, then read the prescription from the patient's old glasses

    Verification starts from the doctor's written prescription because that is the legal specification the eyewear must satisfy; the lab order is then checked as a secondary document for frame, material and treatments. Keeping the spectacle table fixed between the right and left lens preserves the height relationship needed to detect vertical imbalance, and the eyepiece must already have been focused before any power is read.

    Source: American Board of Opticianry exam content outline, dispensing procedures and instrumentation; System for Ophthalmic DispensingReport a problem with this question

19. 19. A finished single-vision job is measured and the cylinder axis is found to be outside the applicable tolerance for its cylinder power, while everything else checks out. What is the correct action?

    • A.Dispense the glasses and note the deviation on the record for the next visit
    • B.Return the job to the laboratory to be remade, because an out-of-tolerance power parameter cannot be corrected by adjusting the frame✓ Answer
    • C.Recenter the lens on the lensmeter stop until the target centers, then re-read the axis
    • D.Rotate the lens slightly in the eyewire until the axis reads correctly and dispense it

    Tolerances define the limit of acceptable manufacturing variation, so a parameter measured outside its tolerance is a defective job and must be remade rather than adjusted or dispensed. Rotating an edged lens in the eyewire changes the axis but also disturbs the fit and any prescribed prism, and recentering on the lens stop only hides the finding instead of correcting it.

    Source: American Board of Opticianry exam content outline, standards and dispensing procedures; ANSI Z80.1 ophthalmic prescription lens standardReport a problem with this question

20. 20. Two jobs are verified: one lens carries 0.50 D of cylinder and another carries 2.50 D of cylinder. Which statement about the axis tolerance that applies to each is correct?

    • A.Axis tolerance depends on the sphere power, not the cylinder power
    • B.The 0.50 D lens must hold its axis within a tighter tolerance, because weak cylinders are harder to grind
    • C.The 2.50 D lens must hold its axis within a tighter tolerance, because the permitted axis deviation narrows as cylinder power rises✓ Answer
    • D.Both lenses share one fixed axis tolerance regardless of cylinder power

    The permitted axis deviation is keyed to cylinder power because a given number of degrees of misalignment produces more residual blur and induced cylinder as the cylinder gets stronger. That is why the tolerance table narrows step by step as cylinder power increases, and it is unrelated to the sphere power of the lens.

    Source: ANSI Z80.1 ophthalmic prescription lens standard, cylinder axis toleranceReport a problem with this question

21. 21. An optician checks the center thickness of a finished plus lens with a thickness caliper and compares it with the nominal value on the lab order. Assume the applicable center-thickness tolerance is plus or minus 0.3 mm and the nominal value is 3.2 mm. Which measured value is out of tolerance?

    • A.2.9 mm
    • B.3.6 mm✓ Answer
    • C.3.0 mm
    • D.3.4 mm

    With a nominal 3.2 mm and a stated tolerance of plus or minus 0.3 mm, any value from 2.9 mm through 3.5 mm is acceptable, so 3.6 mm falls outside the band while 3.4, 3.0 and 2.9 mm all fall inside it. Center thickness matters because it affects both the lens's impact performance and the weight and cosmetics of the finished job.

    Source: ANSI Z80.1 ophthalmic prescription lens standard, center thickness measurement at the prism reference pointReport a problem with this question

Practice questions based on the American Board of Opticianry Basic Certification exam content outline, standard ophthalmic dispensing references, and the federal rules that govern eyewear (the FDA impact-resistance requirement and the FTC Eyeglass and Contact Lens Rules). ABO and NCLE are marks of the American Board of Opticianry and National Contact Lens Examiners; this site is not affiliated with or endorsed by them. Opticianry is licensed in only some states and the requirements differ — confirm your own state board's practice act and the current exam format before testing. About the ABO exam →