22 Instruments & Verification Practice Questions & Answers

1. 1. An optician seats a finished single-vision lens in a manual lensmeter with its concave surface flat against the lens stop. What does the instrument then report, and why is the lens held that way?

   • A.Back vertex power, because the prescription is defined at the rear surface, the one facing the eye✓ Answer
   • B.Equivalent (true) power, because the lens stop removes the effect of centre thickness from the reading
   • C.Front vertex power, because the laboratory grinds the prescription onto the convex side of the blank
   • D.Rear surface power alone, because the stop isolates that one curve the way a lens clock does

   A spectacle lens is prescribed and verified as back vertex power, the power measured from the rear surface toward the eye, so the concave side must rest on the lens stop. A lens clock reads one surface and a lensmeter reads the whole lens, so the two are not interchangeable.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; ANSI Z80.1 definition of back vertex powerReport a problem with this question

2. 2. Two opticians share one manual lensmeter, and the second reads nearly every job about 0.50 D more minus than the first. Nothing else on the instrument has been altered. What explains the difference, and what is the fix?

   • A.The lens stop has worked loose, so it must be reseated to let lenses sit square before reading
   • B.Each operator's accommodation biases the reading, so the eyepiece must be refocused on the reticle✓ Answer
   • C.The internal target has drifted, so the laboratory must reset it against a known standard lens
   • D.The second optician is reading the front surface, so each lens must be turned around on the stop

   If the eyepiece is not focused on the reticle for the person reading, that person's accommodation is stimulated and every power is biased in the same direction. The eyepiece is turned to full plus and then back until the crosshairs snap sharp, with the drum at zero, and it is reset whenever the operator changes.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; standard lensmeter eyepiece focusing procedureReport a problem with this question

3. 3. Reading a lens in minus cylinder on a manual lensmeter, the operator focuses the single sphere lines at -1.75 with the axis wheel at 030, then focuses the close-set lines at -4.25. What is the power of the lens?

   • A.-1.75 -2.50 x 030✓ Answer
   • B.-4.25 -2.50 x 030
   • C.-1.75 -4.25 x 030
   • D.-1.75 -2.50 x 120

   The first clear focus is the sphere and the cylinder is the second focus minus the first, so -4.25 taken from -1.75 gives -2.50 of cylinder, not -4.25. The axis is the one shown when the sphere lines were sharp, which is 030.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; standard minus-cylinder lensmeter reading procedureReport a problem with this question

4. 4. At the lensmeter the single sphere lines of a minus-cylinder lens are in focus and run vertically, from 12 o'clock to 6 o'clock. What does the axis wheel read, and what follows from it?

   • A.Axis 180, so the cylinder power lies in the vertical 090 meridian✓ Answer
   • B.Axis 180, since the sphere lines are named for the meridian they cross
   • C.Axis 090, so the cylinder power lies in that same vertical meridian
   • D.Axis 090, so the cylinder power lies in the horizontal 180 meridian

   In standard notation the axis is the meridian the sphere lines lie along, so vertical sphere lines read 090 and horizontal sphere lines read 180. Cylinder power always acts 90 degrees from its axis, so an axis of 090 puts the cylinder in the 180 meridian.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; standard (TABO) axis notation 0-180Report a problem with this question

5. 5. As the power drum is turned, the single lines and the close-set lines of a lens come sharp together at -3.00, and they stay sharp through a full turn of the axis wheel. What does this tell the verifier?

   • A.The eyepiece is still unfocused, so the two sets have merged falsely
   • B.The lens holds -3.00 of cylinder at an axis the wheel cannot resolve
   • C.The lens is a -3.00 sphere, so there is just one power to be found✓ Answer
   • D.The lens sits crooked on the stop, which blurs both sets of lines alike

   Cylinder exists only when the two sets of lines focus at two different drum settings, the difference between them being the cylinder power. One common focus that survives rotation of the axis wheel means both principal meridians carry the same power, which is a sphere.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; standard lensmeter sphere and cylinder identificationReport a problem with this question

6. 6. A flat-top bifocal in a plus prescription is being verified. With the back surface on the stop the distance portion reads +2.00 and the segment reads +4.10, so the optician records a +2.10 add against the +2.00 ordered. What is wrong here?

   • A.A front-surface segment is verified as front vertex power, both readings taken convex side down✓ Answer
   • B.The lens should be read in plus cylinder first, since the add is taken from the flatter meridian
   • C.The technique is sound, so the job is 0.10 D over the order and should go back for remake
   • D.The segment must be read at its own optical centre, well below the line where this reading was taken

   A conventional segment ground on the front surface must be verified as front vertex power, so the lens is turned around and both the distance and the segment readings are taken with the convex surface against the stop. Read in back vertex orientation the add is overstated, and the error grows with plus power.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; front vertex verification of front-surface multifocal addsReport a problem with this question

7. 7. A progressive lens has been remarked from its permanent engravings, which sit 34 mm apart. Which reference is used for the distance power, and which for prescribed prism?

   • A.Distance power at the fitting cross; prism halfway between the two engraved circles
   • B.Distance power in the near reference circle; prism at the geometric centre of the lens
   • C.Distance power in the distance verification circle; prism at the prism reference point✓ Answer
   • D.Distance power at the prism reference point; prism in the distance verification circle

   The remarking chart gives a distance verification circle above the fitting cross for distance power, a near reference circle for the add, and a prism reference point midway between the engravings for prism. Prism must be read at that point because it is the one location where the design's prescribed prism is specified.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; manufacturer remarking charts for progressive lens verificationReport a problem with this question

8. 8. A lens is ordered with 2 prism dioptres base out. At the lensmeter the target sits off to one side of the reticle centre, and a trainee slides the lens across the stop until the target is centred, then records no prism. What is the error?

   • A.Sliding the lens hides the prism, which is read as target displacement with the lens held as worn✓ Answer
   • B.Base-out prism displaces the target vertically, so the wrong meridian of the reticle was watched
   • C.Prism is read only from the power drum, so the position of the target tells the verifier nothing
   • D.The rings read in millimetres, not prism dioptres, so the trainee should have divided by ten

   Prism is measured by how far the target has moved away from the centre of the reticle while the lens is held in the position it will occupy when worn, one ring being one prism dioptre. Moving the lens until the target is centred simply places the optical centre on the axis of the instrument and erases the very prism being checked.

   Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; standard lensmeter prism measurement procedureReport a problem with this question

9. 9. A wearer whose distance PD is 62 mm reports eyestrain with new glasses. Verification finds the optical centres of the two -4.00 D lenses 68 mm apart. What prismatic effect is the wearer receiving at distance?

   • A.Base in, 1.2 prism dioptres in total, shared by the two lenses
   • B.Base out, 2.4 prism dioptres in total, 1.2 from each lens
   • C.Base in, 0.24 prism dioptres in total, from 3 mm of decentration
   • D.Base in, 2.4 prism dioptres in total, 1.2 from each lens✓ Answer

   Each centre is 3 mm temporal to its line of sight, and Prentice's rule gives 4.00 times 0.3 cm, which is 1.2 prism dioptres per eye, 2.4 in total. In a minus lens the base lies away from the optical centre, so a centre placed temporally produces base-in prism; the distance must be converted to centimetres, not left in millimetres.

   Source: Prentice's rule as applied in ophthalmic dispensing (prism dioptres = lens power in dioptres x decentration in centimetres)Report a problem with this question

10. 10. A lens carries so much base-in prism that the lensmeter target falls beyond the edge of the reticle. The verifier holds a 5 prism dioptre base-out auxiliary prism at the stop, and the target then rests two rings toward the nose. What prism does the lens carry?

    • A.3 prism dioptres base in: the 2 still showing taken from the 5 of the auxiliary prism
    • B.2 prism dioptres base in: the auxiliary prism is a viewing aid and is not counted in
    • C.7 prism dioptres base in: the 2 still showing added to the 5 of the auxiliary prism✓ Answer
    • D.5 prism dioptres base out: read straight from the auxiliary prism used to recentre it

    An auxiliary prism is used when the amount exceeds the range of the reticle, and it is oriented with its base opposite to the prism in the lens so the target is dragged back into view. The residual displacement is then added to the value of the auxiliary prism, so 2 plus 5 gives 7 prism dioptres base in.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens power measuring devices; auxiliary (compensating) prism procedure for out-of-range prismReport a problem with this question

11. 11. Moving an unknown lens across a distant vertical line at arm's length, the optician sees the line travel opposite to the lens. Trial lenses are then held with it until the line stops moving. What is the lens, and what neutralises it?

    • A.A minus lens; plus trial lenses are added until the motion stops
    • B.A prism; a trial prism of equal power stops the apparent motion
    • C.A plus lens; plus trial lenses are added until the motion stops
    • D.A plus lens; minus trial lenses are added until the motion stops✓ Answer

    A plus lens forms an inverted image of a distant object, so the target appears to move against the lens, while a minus lens gives with motion. Neutralisation works by stacking a trial lens of the opposite sign until the combination has no power and no apparent movement remains; the neutralising lens equals the unknown lens in magnitude.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Ophthalmic Optics domain: hand neutralization of spectacle lenses with trial lensesReport a problem with this question

12. 12. A lens clock is set on the front surface of a lens and rotated through a full sweep at the same spot. The reading changes from +6.00 at one position to +8.00 ninety degrees away. What does this show about that surface?

    • A.It is toric, carrying 2.00 D of surface cylinder between its principal meridians✓ Answer
    • B.It is spherical, and the change comes from the centre pin riding over a coating
    • C.It is aspheric, flattening steadily from its centre out toward the lens edge
    • D.It is warped, because a sound surface holds one reading right through the sweep

    A lens clock reads the curvature of one surface between its fixed outer pins and its moving centre pin, so a spherical surface gives the same value in every direction at a given spot. Two different readings ninety degrees apart identify a toric surface, and the difference between them is the surface cylinder.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens measurement devices (lens clock)Report a problem with this question

13. 13. A lens clock calibrated for an index of 1.530 reads -6.00 D on the back surface of a polycarbonate lens, whose index is 1.586. What is the true power of that surface?

    • A.-6.00 D exactly, since the clock reads curvature only
    • B.About -5.43 D, the reading scaled by 0.530/0.586
    • C.About -6.22 D, the reading scaled by 1.586/1.530
    • D.About -6.63 D, the reading scaled by 0.586/0.530✓ Answer

    Surface power is (n - 1) divided by the radius, and the dial is engraved for n = 1.530, so the true value is the reading multiplied by (n actual - 1) divided by 0.530. For polycarbonate that factor is 0.586/0.530, about 1.11, giving roughly -6.63 D.

    Source: Surface power formula F = (n - 1)/r with the lens clock's standard 1.530 calibration indexReport a problem with this question

14. 14. A lens clock reads +6.25 on the front surface and -9.00 on the back surface of a finished lens. What may the verifier conclude from these two readings, and what is still required?

    • A.A verified power of -2.75, so no lensmeter reading of this lens is needed
    • B.An approximate power near -2.75; the lensmeter must still give the verified power✓ Answer
    • C.An approximate power near -15.25, found by subtracting one surface from the other
    • D.A base curve of -2.75, which is the figure the laboratory order itself specifies

    Adding the front and back surface readings gives nominal or approximate power, +6.25 with -9.00 being about -2.75, which is useful as a check but ignores centre thickness and the position of the two surfaces. Only the lensmeter gives back vertex power, which is the value the prescription and the tolerances refer to.

    Source: Nominal (approximate) lens power as the sum of front and back surface powers; ANSI Z80.1 tolerances applied to measured back vertex powerReport a problem with this question

15. 15. A patient has an irregular, off-centre pupil in one eye. A corneal reflex pupillometer gives 32/31 monocular and a millimetre rule gives 64 total. Which figures should be used for a progressive job, and why?

    • A.The rule value, because it is taken through the very frame the patient is going to wear
    • B.The pupillometer values, taken from the corneal light reflex rather than the pupil margin✓ Answer
    • C.The rule value, because a total distance PD is what the laboratory decentres each lens from
    • D.Either set, since a 1 mm difference stays well inside the usual decentration tolerance

    A corneal reflex pupillometer measures from the first Purkinje reflex on each cornea, so an irregular or displaced pupil does not move the reading, and it gives monocular values. A progressive lens is centred eye by eye, so two monocular figures are needed rather than one total split in half.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: dispensing instrumentation (corneal reflex pupillometer)Report a problem with this question

16. 16. A -9.00 D prescription was refracted at a 13 mm vertex distance, and a distometer shows the chosen frame will hold the lens at 9 mm. What does that instrument measure, and what does the finding call for?

    • A.Corneal apex to back lens surface; a change of this size may safely be ignored
    • B.Closed lid to front lens surface; the frame must be bent out to restore the 13 mm
    • C.Pupil centre to the frame plane; the laboratory compensates from that figure alone
    • D.Corneal apex to back lens surface; the 4 mm change calls for compensating the power✓ Answer

    A distometer reads the distance from the corneal apex to the back surface of the lens, usually over the closed lid with an allowance for lid thickness. Effective power at the eye changes once a meridian reaches about four dioptres and the vertex moves more than about two millimetres, and the compensated power is found from F divided by (1 - dF) with d in metres.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: dispensing instrumentation (distometer); vertex compensation formula Fc = F/(1 - dF)Report a problem with this question

17. 17. An order specifies a 1.8 mm centre thickness, and the centre thickness tolerance in use is plus or minus 0.3 mm. A thickness caliper reads 2.2 mm at the optical centre of the finished lens. What is the correct disposition?

    • A.Dispense it; centre thickness carries no tolerance at all on a finished lens
    • B.Return it; at 0.4 mm over the order it falls outside the stated 0.3 mm allowance✓ Answer
    • C.Dispense it; 0.4 mm is within 0.3 mm once that figure is read as a percentage
    • D.Return it; any centre thickness above 2.0 mm fails the impact-resistance rule

    Centre thickness is measured with a thickness caliper or gauge at the optical centre and compared with the figure on the laboratory order. The lens is 0.4 mm thicker than ordered, which exceeds the plus or minus 0.3 mm allowance given in the stem, so the job is returned rather than dispensed.

    Source: ANSI Z80.1 center thickness tolerance (tolerance value supplied in the stem); use of a thickness caliper at final inspectionReport a problem with this question

18. 18. A segment height of 18 mm is ordered on a flat-top bifocal. On the finished lens, from what reference is that 18 mm measured, and to what?

    • A.From the lowest point of the boxed lens shape up to the optical centre of the lens
    • B.From the geometric centre of the lens down to the top line of the segment itself
    • C.From the lowest point of the boxed lens shape up to the top line of the segment✓ Answer
    • D.From the bottom of the eyewire groove up to the wearer's own lower lid margin

    Segment and optical centre heights are taken in the boxing system, from the lowest point of the boxed lens shape, which is the bottom of the bevel, up to the point in question. For a flat-top bifocal that point is the top line of the segment; the lower lid margin is where the segment is fitted on the patient, not the reference on the lens.

    Source: Boxing system as used for segment and optical center height measurement; ANSI Z80.1 segment height verificationReport a problem with this question

19. 19. An order reads -2.00 -1.75 x 175 and the lensmeter gives -2.00 -1.75 x 172. The axis table in use allows 14 degrees at 0.25 D of cylinder, 7 degrees to 0.50 D, 5 degrees to 0.75 D, 3 degrees to 1.50 D and 2 degrees above 1.50 D. Does the job pass?

    • A.Yes; the 3 degree step applies to this lens and the axis is off by exactly that much
    • B.No; the axis has to match the order exactly whenever any cylinder has been ordered
    • C.Yes; the allowance widens as cylinder power rises, so the 14 degree step applies
    • D.No; above 1.50 D of cylinder only 2 degrees are allowed and the axis is off by 3✓ Answer

    Cylinder axis tolerance is a function of cylinder power and tightens as the cylinder grows, because a given axis error costs more optically in a strong cylinder than in a weak one. At 1.75 D the lens falls in the band above 1.50 D, where the stem allows 2 degrees, and an error of 3 degrees fails.

    Source: ANSI Z80.1 (2020 edition) cylinder axis tolerance table; the tolerance values are supplied in the stemReport a problem with this question

20. 20. A digitally surfaced progressive arrives with a list of compensated powers that differ from the written prescription. At the lensmeter the lens matches the compensated figures but not the written ones. How should the job be handled?

    • A.Reject it; a finished lens must always read the prescriber's written power at the bench
    • B.Accept it; every progressive design is exempt from power tolerances because of its corridor
    • C.Accept it; tolerances apply to the compensated power once a lens is position-compensated✓ Answer
    • D.Reject it; a laboratory may not alter a written power without the prescriber's consent

    When a lens is compensated for the position of wear, the surfaced power is deliberately different so that the power reaching the eye through the tilted, vertex-set lens equals the prescription. The standard therefore states that verification is carried out against the compensated values supplied by the laboratory, not against the written prescription.

    Source: ANSI Z80.1 (2020 edition) provision that tolerances apply to the compensated power for lenses compensated for position of wearReport a problem with this question

21. 21. An optician carries out final inspection on a finished pair before dispensing. Which sequence follows the standard order of that inspection?

    • A.Cosmetic and frame checks, then power and axis, then centre placement, then add and prism
    • B.Power and axis, add, prism, centre placement and heights, then cosmetic and frame checks✓ Answer
    • C.Frame alignment, then centre placement and heights, then cosmetic checks, then power and axis
    • D.Centre placement and heights, then cosmetic checks, then frame alignment, then power and add

    Final inspection works from the optics outward: the prescription values are confirmed first at the major reference point, then the add and any prism, then the placement of the centres and the segment or fitting cross heights, and only then the cosmetic condition and standard bench alignment of the frame. Checking cosmetics or alignment first wastes work on a pair that may have to go back for power.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Dispensing Procedures domain: final inspection and verification sequenceReport a problem with this question

22. 22. At final inspection an optician must confirm the base curve actually put on a lens, the perimeter of the lens against the eyewire groove, and the wearer's vertex distance. Which instruments do those three jobs, in that order?

    • A.Lensmeter, thickness caliper, pupillometer
    • B.Lens clock, circumference gauge, distometer✓ Answer
    • C.Radiuscope, circumference gauge, keratometer
    • D.Lens clock, thickness caliper, pupillometer

    Base curve is a surface measurement, so it is read with a lens clock; the perimeter of a lens, pattern or eyewire groove is read with a circumference gauge; and the corneal apex to lens distance is read with a distometer. The radiuscope and the keratometer belong to contact lens work and measure a lens or corneal radius, not a spectacle base curve.

    Source: ABO-NCLE Basic Exam (NOCE) test specifications, Instrumentation domain: use of lens measurement devices and dispensing instrumentationReport 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 →