NCLE Practice Test: Contact Lens Exam Questions With Worked Answers

The NCLE Basic exam is the Contact Lens Registry Examination (CLRE): a secure, computer-based test of 125 multiple-choice questions in 2 hours, where a portion of the questions may be unscored pretest items. The content outline scores 100 of them across eight domains. ABO-NCLE sells an official practice exam. This page offers something different: 24 original questions weighted by the published outline, every calculation shown step by step, and an answer key.

For eligibility, fees and scoring, see ABO and NCLE certification. For the spectacle-side exam, use the ABO practice test. If you are still choosing a training route, start with how to become an optician.

These questions are original practice items in the style of the official outline, not real exam items, and none come from ABO-NCLE or any prep site. Cover the answer, try each question, then read the working. Domain names and scored-question counts come from the ABO-NCLE Basic Exam Handbook (February 2025). Questions about exam rules cite the Contact Lens Rule as it reads on the eCFR (16 CFR Part 315).

How This Practice Set Maps to the CLRE

Practice counts follow each domain’s share of the 100 scored questions, rounded to total 24. Condition and pathology items are left out on purpose.

Rules Used in Every Calculation

Three conventions are fixed here so no question depends on a guess about the method.

Vertex conversion. Contact lens power Fc = F / (1 – d × F), with d in meters. The vertex distance is 12 mm (0.012 m) unless a stem says otherwise. Round to the nearest 0.25 D, each meridian separately for a toric prescription. The contact lens power converter and the vertex distance calculator run the same formula, and vertex distance compensation explains why it matters.

Radius and power. K (D) = 337.5 / r (mm), which is the keratometer scale built on an index of 1.3375. A smaller radius is a steeper curve.

Toric rotation (LARS). The University of Iowa’s EyeRounds atlas states that you can compensate the axis with “the LARS rule- Left Add, Right Subtract,” and its example is a lens “rotated about 15 degrees to the doctor’s left,” where an original axis of 020 becomes 035. The AOP’s article on torics names the same rule as “Clockwise Add, Anti-clockwise Subtract (CAAS)” or “Left Add, Right Subtract” (LARS).

Bausch + Lomb’s fitting guide for PureVision2 For Astigmatism gives the same rule in clock terms: “if the rotation is clockwise but stable, note the amount of rotation, add it to the refractive cylinder axis.” Study notes sometimes mix the patient’s view with the examiner’s, so this page defines it once:

The clockwise and counterclockwise labels are geometry for a marking at 6 o’clock, as seen by the examiner facing the patient. If a lens uses a different marking position, confirm the manufacturer’s convention before ordering. Every rotation question below uses a 6 o’clock marking and says which way it moved.

Ocular Anatomy, Physiology, and Pathology (12 of 100 scored questions)

The handbook lists the tear film, cornea, lids and conjunctiva under this domain. These questions stay on structure and function.

Question 1: Layers of the tear film

Which layer forms the outer surface of the tear film, and which structures produce it?

• A. Aqueous layer, produced by the lacrimal gland
• B. Lipid layer, produced by the meibomian glands
• C. Mucin layer, produced by the goblet cells
• D. Lipid layer, produced by the lacrimal gland

Answer: B. The tear film has three layers: a lipid layer on the outside, an aqueous layer in the middle, and a mucin layer against the cornea. The meibomian glands in the lid margins supply the lipid layer, which slows evaporation of the layer beneath it. Options A and C name the middle and inner layers. Option D has the right layer but the wrong gland: the lacrimal gland supplies the aqueous layer.

Question 2: Oxygen supply with the lids closed

During sleep, where does the cornea get most of its oxygen?

• A. Directly from the air through the closed lids
• B. From the vitreous behind the lens
• C. From the blood vessels of the palpebral conjunctiva
• D. From the tears, which are replaced every few seconds

Answer: C. The cornea has no blood vessels of its own. With the eye open it takes oxygen mainly from the air dissolved in the tear film. With the lids closed, the blood vessels lining the inside of the upper lid (palpebral conjunctiva) become the main source, and the supply is lower than in the open eye. Option A names the air route, which needs the eye open, B is not a corneal oxygen source, and D describes tear turnover, which is not what delivers oxygen when the lids are shut.

Question 3: Where the eye’s refractive power comes from

Which corneal surface contributes the largest part of the eye’s refractive power, and why does that matter for contact lens fitting?

• A. The anterior surface, because the air-tear interface has the largest refractive index step
• B. The posterior surface, because it touches the aqueous
• C. The endothelium, because it is the thinnest layer
• D. Bowman layer, because it is the strongest layer

Answer: A. Light passes from air into the tear film and cornea, the largest change in refractive index along the whole optical path, so the front surface does most of the refracting. A keratometer measures the curvature of that front surface, and a contact lens works by replacing it with a new front surface of its own.

Refractive Errors (5 of 100 scored questions)

The handbook includes the lacrimal (tear) lens and the sagittal depth effects of base curve and diameter in this domain.

Question 4: Tear lens compensation with a rigid lens

A patient’s refraction at the corneal plane is -3.75 D sphere. The flat keratometer reading is 42.25 D. You trial a rigid gas permeable (RGP) lens with a base curve of 43.00 D. Using SAM-FAP (Steeper Add Minus, Flatter Add Plus), which lens power is closest?

• A. -3.00 D
• B. -4.50 D
• C. -3.75 D
• D. -5.25 D

Answer: B. The lens is 43.00 – 42.25 = 0.75 D steeper than flat K. A steeper lens vaults the center of the cornea and traps a plus tear lens of about +0.75 D. Cancel it by adding minus power: -3.75 + (-0.75) = -4.50 D. Check: -4.50 D plus +0.75 D gives -3.75 D. Option A applies the flatter rule, C ignores the tear lens, and D doubles the tear lens compensation.

Instrumentation for Measurement and Observation (12 of 100 scored questions)

Question 5: Keratometer readings and corneal astigmatism

A keratometer reads 43.00 D at 180 and 44.50 D at 090. What does this show?

• A. 1.50 D of with-the-rule corneal astigmatism
• B. 1.50 D of against-the-rule corneal astigmatism
• C. 0.75 D of with-the-rule corneal astigmatism
• D. 1.50 D of oblique corneal astigmatism

Answer: A. Corneal astigmatism is the difference between the two principal meridians: 44.50 – 43.00 = 1.50 D. The steeper meridian is 090 (vertical), so the cornea is steepest vertically, which is with-the-rule. The flat meridian is horizontal, at 180. Option B reverses which meridian is steeper, C halves the difference (a spherical equivalent step, not a cylinder), and D calls it oblique, which applies only when the principal meridians sit away from 90 and 180. Here they sit exactly on them. In millimeters the readings are 337.5 / 43.00 = 7.85 mm and 337.5 / 44.50 = 7.58 mm.

Question 6: Lighting for fluorescein

Which slit lamp illumination makes fluorescein glow so you can read a fluorescein pattern under a rigid lens?

• A. Diffuse white light
• B. Red-free (green) filter
• C. Retroillumination
• D. Cobalt blue filter

Answer: D. Fluorescein absorbs blue light and emits green-yellow light, so the cobalt blue filter is the one that makes the dye visible. The other settings do not make the dye fluoresce strongly enough to judge pooling or touch.

Question 7: Timing tear film break-up time

Tear film break-up time (BUT) is timed from which point to which point?

• A. From a complete blink to the first dark break in the stained tear film
• B. From fluorescein instillation to the first blink
• C. From lid opening after the test strip is removed to the first blink
• D. From the first dark break to the second dark break

Answer: A. The patient blinks completely and then stares ahead without blinking. You watch the stained tear film under cobalt blue light and stop the clock at the first dark spot, line or break. Option B confuses staining with timing, C times the interval until the next blink, not the break-up of the tear film, and D measures nothing the outline defines.

Prefitting (15 of 100 scored questions)

Question 8: Spectacle to contact lens power for a toric prescription

A patient’s spectacle prescription is -4.00 -2.00 x 180 at a vertex distance of 12 mm. Convert each meridian and round each to the nearest 0.25 D. Which contact lens power is closest?

• A. -4.00 -2.00 x 180
• B. -3.75 -1.75 x 180
• C. -4.25 -2.25 x 180
• D. -3.75 -2.00 x 180

Answer: B. Convert the two principal meridians, then rebuild the cylinder. The first meridian is -4.00 D: Fc = -4.00 / (1 – 0.012 × -4.00) = -4.00 / 1.048 = -3.82 D, which rounds to -3.75 D. The second meridian is -4.00 + (-2.00) = -6.00 D: Fc = -6.00 / (1 – 0.012 × -6.00) = -6.00 / 1.072 = -5.60 D, which rounds to -5.50 D. The cylinder is -5.50 – (-3.75) = -1.75 D, so the contact lens is -3.75 -1.75 x 180. Option A skips conversion, C uses F / (1 + dF), the reverse formula that takes a contact lens power back to the spectacle plane (F / (1 – dF) applies to plus and minus lenses alike), and D converts the sphere but leaves the cylinder unchanged. The contact lens power converter accepts a full toric prescription.

Question 9: Choosing a rigid base curve from the keratometer

The flat K reads 7.80 mm. You want a rigid base curve 0.50 D steeper than flat K. Which base curve, in millimeters to two decimals, do you order?

• A. 7.71 mm
• B. 7.89 mm
• C. 7.80 mm
• D. 7.30 mm

Answer: A. Convert to diopters: 337.5 / 7.80 = 43.27 D. Add 0.50 D for a steeper fit: 43.27 + 0.50 = 43.77 D. Convert back: 337.5 / 43.77 = 7.71 mm. Option B subtracts 0.50 D (a flatter fit, 337.5 / 42.77 = 7.89 mm), C is “on K”, and D subtracts 0.50 mm instead of 0.50 D. A 0.50 D change is worth about 0.09 mm at this radius, not 0.50 mm.

Question 10: Comparing oxygen transmissibility

Lens A has a Dk of 60 and a center thickness of 0.10 mm. Lens B has a Dk of 100 and a center thickness of 0.20 mm. Dk is in the usual units (× 10⁻¹¹) and Dk/t is reported in the usual units (× 10⁻⁹). Which statement is correct?

• A. Lens B transmits more oxygen, because its Dk is higher
• B. Lens A has Dk/t of 6 and lens B has Dk/t of 20
• C. Lens A has the higher Dk/t: 60 against 50
• D. The two lenses have equal Dk/t

Answer: C. Transmissibility is permeability divided by thickness. The National Academies proceedings on contact lens use say it directly: “the Dk value is divided by the lens thickness (L) to give a Dk/L value, which is called transmissibility.” In those units the thickness goes in centimeters: 0.10 mm is 0.010 cm and 0.20 mm is 0.020 cm. Lens A: 60 / 0.010 = 6,000 × 10⁻¹¹, written as 60 × 10⁻⁹. Lens B: 100 / 0.020 = 5,000 × 10⁻¹¹, written as 50 × 10⁻⁹. A quick check gives the same numbers: divide Dk by the thickness in millimeters, then by 10. Lens A passes more oxygen despite the lower Dk, because it is half as thick. Option A compares Dk alone, B multiplies Dk by thickness (60 × 0.10 = 6, 100 × 0.20 = 20), and D assumes thickness cancels out.

Diagnostic Fitting (11 of 100 scored questions)

Question 11: Compensating a rotated toric soft lens

A toric soft lens ordered as -2.50 -1.75 x 170 settles with its 6 o’clock marking rotated 20 degrees toward the examiner’s left (clockwise as the examiner sees it). Using LARS as defined above, which axis do you order?

• A. 150
• B. 170
• C. 190
• D. 010

Answer: D. Left means add: 170 + 20 = 190. Axes run from 0 to 180 degrees, so 190 wraps around to 190 – 180 = 010. Option A subtracts, B ignores the rotation, and C leaves an impossible axis number.

Question 12: Reading a rigid lens fluorescein pattern

Under cobalt blue light, a rigid lens shows a dark center where it touches the cornea and a wide band of bright fluorescein pooling at the edge. What is the fit, and what change helps?

• A. Flat fit; order a steeper base curve
• B. Flat fit; order a flatter base curve
• C. Steep fit; order a flatter base curve
• D. Steep fit; order a steeper base curve

Answer: A. Central touch with excess edge clearance means the lens is flatter than the cornea, so it bears on the center and lifts at the edge. The correction is a steeper base curve (a smaller radius). A steep fit shows the opposite: pooling under the center and touch at the periphery. Option B would flatten an already flat lens further, and C and D misread the pattern.

Question 13: Sphere power from an overrefraction

A diagnostic soft sphere of -5.50 D gives an overrefraction of -0.75 -0.50 x 180. The residual cylinder is small, and you will order a spherical lens. The power is small enough that vertex effects can be ignored. Which lens power do you order?

• A. -6.75 D
• B. -6.50 D
• C. -6.25 D
• D. -5.75 D

Answer: B. Replace the overrefraction with its spherical equivalent: -0.75 + (-0.50 / 2) = -0.75 + (-0.25) = -1.00 D. Add that to the diagnostic lens: -5.50 + (-1.00) = -6.50 D. Option A adds the full cylinder (-5.50 – 0.75 – 0.50 = -6.75), C uses only the sphere of the overrefraction, and D applies only half the cylinder (-0.25 D) and drops the sphere of the overrefraction.

Dispensing (20 of 100 scored questions)

Question 14: Hydrogen peroxide systems

A patient disinfects soft lenses in a hydrogen peroxide system. What must happen before the lenses go on the eye?

• A. Nothing, because the lenses are disinfected and ready
• B. The lenses must be rinsed with tap water
• C. A few drops of multipurpose solution must be added to the peroxide
• D. The peroxide must be neutralized, using the neutralizing step of that system

Answer: D. Hydrogen peroxide disinfects only while it is still peroxide. It has to be converted to a harmless solution by the system’s neutralizer (a platinum disc in the lens case or a tablet, depending on the product) before the lens is worn. Teach patients not to wear a lens until the system’s neutralization step is complete. Option A skips that step, B uses water, and C mixes products not designed to be mixed.

Question 15: Contact lens case care

Which routine matches the CDC’s contact lens hygiene guidance for the case?

• A. Add fresh solution to what is already in the case, rinse with tap water, replace yearly
• B. Empty the case, keep it closed and wet between uses, replace when it looks worn
• C. Rub and rinse the case with solution, store it upside down with caps off, replace at least every three months
• D. Rinse with hot tap water weekly, replace every six months

Answer: C. The CDC page on healthy contact lens wear says to “Clean case with solution by rubbing and rinsing,” to “Store case upside down with caps off after each use,” and to “Replace case at least once every three months.” It also says to “Avoid mixing fresh and old solution in the case,” which rules out option A. B keeps the case wet and has no fixed replacement interval, and D uses water and a six-month interval.

Question 16: Telling a soft lens is inside out

You balance a soft lens on your fingertip and look at it from the side. The edge flares outward like a bowl with a lip. What do you conclude?

• A. The lens is inside out and should be turned over
• B. The lens is the right way round
• C. The lens is the wrong power
• D. The lens is too dry to evaluate

Answer: A. A lens that is right side out forms a smooth bowl with the edges pointing straight up or curling slightly inward. Flared, outward-turned edges mean the lens is inverted. The lens should be reversed before insertion. Wrong power (C) and dryness (D) do not change the edge profile on the fingertip.

Question 17: Water and contact lenses

A patient asks whether it is acceptable to leave monthly soft lenses in during a shower. What is the best answer, per the CDC?

• A. Yes, because a shower is brief and the water is clean
• B. Only swimming pools are a concern, not showers
• C. Remove lenses before swimming or showering
• D. Yes, as long as the lenses are rinsed in tap water afterward

Answer: C. The CDC guidance says: “Remove before swimming or showering to prevent germ exposure.” It also says to use fresh solution to store lenses, “never water.” Options A and B treat shower water as safe, and D adds a tap water rinse, which is the opposite of the advice.

Question 18: Overwear of a replacement lens

A patient is prescribed 30-day monthly lenses and wears one pair for 6 weeks (42 days). By what percentage did the patient exceed the replacement interval?

• A. 40%
• B. 12%
• C. 50%
• D. 140%

Answer: A. Overage is the days beyond the schedule divided by the schedule: (42 – 30) / 30 = 12 / 30 = 0.40, or 40%. Option B is the number of extra days, C takes 6 weeks against 4 weeks, and D is the total wear as a percentage of the schedule (42 / 30 = 140%), not the excess.

Follow-Up (20 of 100 scored questions)

Question 19: Sagittal depth when the diameter changes

An RGP lens has a base curve radius of 7.80 mm. Ignoring peripheral curves, use sagittal depth s = r – sqrt(r² – (d / 2)²). What is the sagittal depth at diameters of 9.0 mm and 9.5 mm, and what does the larger diameter do to the fit?

• A. 1.61 mm at 9.0 mm and 1.43 mm at 9.5 mm; the fit loosens
• B. 1.43 mm at both diameters; no change
• C. 1.43 mm at 9.0 mm and 1.61 mm at 9.5 mm; the fit tightens
• D. 1.43 mm at 9.0 mm and 1.61 mm at 9.5 mm; the fit loosens

Answer: C. At 9.0 mm: sqrt(7.80² – 4.50²) = sqrt(60.84 – 20.25) = sqrt(40.59) = 6.371, so s = 7.80 – 6.371 = 1.43 mm. At 9.5 mm: sqrt(60.84 – 4.75²) = sqrt(60.84 – 22.5625) = sqrt(38.2775) = 6.187, so s = 7.80 – 6.187 = 1.61 mm. A larger diameter on the same base curve raises sagittal depth by 0.18 mm, which makes the lens fit tighter. Option A swaps the values, B ignores diameter, and D has the values right but reads them as a looser fit.

Question 20: A toric lens that will not hold still

A toric soft lens settles 15 degrees toward the examiner’s left at one visit and 10 degrees toward the examiner’s right at the next, with the marking at 6 o’clock both times. What is the sensible next step?

• A. Add 15 degrees to the axis and reorder the same lens
• B. Subtract 10 degrees from the axis and reorder the same lens
• C. Average the two and add 2.5 degrees to the axis
• D. Do not compensate; try a different lens design or brand with better stability

Answer: D. LARS only works when the lens settles in the same place every time. EyeRounds puts it plainly: “If the location of the laser marking is unstable, you cannot compensate for that.” Here the rotation changes direction between visits, so any single offset would be wrong at one of them. Options A, B and C apply a fixed correction to a lens with no fixed rotation.

Question 21: Counterclockwise rotation

A toric soft lens ordered as -1.75 -1.25 x 080 settles with its 6 o’clock marking rotated 10 degrees counterclockwise as the examiner sees it (toward the examiner’s right). Which axis do you order?

• A. 090
• B. 080
• C. 070
• D. 170

Answer: C. The marking moved toward the examiner’s right, so Right Subtract: 80 – 10 = 70, an axis of 070. Option A adds when it should subtract, B ignores the rotation, and D shifts the axis by 90 degrees, which would be a transposition step and not a rotation fix.

Question 22: Power change when the rigid base curve changes

An RGP lens has a base curve of 7.70 mm and a power of -4.00 D. The overrefraction is -0.50 D. You refit with a 7.85 mm base curve. Round the base curve change to the nearest 0.25 D and the final power to the nearest 0.25 D. Which power do you order?

• A. -3.50 D
• B. -4.50 D
• C. -5.25 D
• D. -3.75 D

Answer: D. Power needed with the old base curve: -4.00 + (-0.50) = -4.50 D. Base curves in diopters: 337.5 / 7.70 = 43.83 D and 337.5 / 7.85 = 42.99 D. The new lens is 43.83 – 42.99 = 0.84 D flatter, about 0.75 D to the nearest 0.25. Flatter means add plus: -4.50 + 0.75 = -3.75 D. Using the unrounded 0.84 D gives -3.66 D, which also rounds to -3.75 D. Option A over-corrects the change, B ignores it, and C adds minus, which is the “steeper” rule applied to a flatter lens.

Question 23: Documenting a parameter change

At a follow-up you change the base curve and power of a patient’s lenses. Which record entry lets another practitioner reproduce the fit?

• A. “Lenses changed, patient happy”
• B. New base curve and power only
• C. Lens brand, base curve, diameter, power (and axis for torics), care system, the reason for the change, and the date
• D. A copy of the supplier invoice

Answer: C. The handbook lists documenting patient records under the Follow-Up plan, and its Regulatory and Administrative domain covers charts that identify changes in lens parameters and care system. Option A has no parameters, B omits diameter, brand and care system, and D shows what was bought, not what was fitted.

Regulatory and Administrative (5 of 100 scored questions)

Question 24: The eight-business-hour verification window

A seller sends a complete verification request to a prescriber, who receives it on Friday at 3:00 p.m. (prescriber’s time zone). The seller has not opted to count Saturday hours, and the following Monday is not a federal holiday. By when must the prescriber respond before the seller can treat the prescription as verified?

• A. Friday at 11:00 p.m.
• B. Monday at 11:00 a.m.
• C. Monday at 3:00 p.m.
• D. Tuesday at 9:00 a.m.

Answer: C. The Contact Lens Rule defines a business hour as “an hour between 9 a.m. and 5 p.m., during a weekday (Monday through Friday), excluding Federal holidays.” It adds that eight business hours are “calculated from the time the prescriber receives the prescription verification information from the seller.” Count from Friday 3:00 p.m. to 5:00 p.m., which is 2 hours. That leaves 8 – 2 = 6 hours. Business hours resume Monday at 9:00 a.m., so 6 more hours ends at 3:00 p.m. Monday. Under 16 CFR 315.5(c)(3), a prescription is verified if the prescriber “fails to communicate with the seller within eight (8) business hours.” Option A counts clock hours, B stops after only 2 hours of Monday, and D waits an extra business day.

NCLE Answer Key

Two More Contact Lens Rule Facts

16 CFR 315.3(a)(1) says that “Whether or not requested by the patient,” the prescriber “shall provide to the patient a copy of the contact lens prescription.” 16 CFR 315.6 sets the expiration floor at one year unless the prescriber documents a medical reason for a shorter period, and the FTC’s guide for prescribers and sellers confirms that the rule “sets a floor, or minimum, expiration date of one year.”

Frequently Asked Questions

Using the Official Resources

Use the handbook outline as your study map and weight your time by each domain’s scored-question count. ABO-NCLE’s own practice exam shows its item style. A state licensing requirement supersedes ABO-NCLE’s when the two differ.

Saulo Garcia

I grew up inside an optical shop. My mother likes to tell how, as a kid, I would watch eyeglasses being assembled, play with the tools, and draw on the back of service order slips. The family business taught me early what a precise measurement and work done right are worth.
I went on to spend more than twenty years as a software engineer, always keeping an eye on the optical world. Optogrid was born where the two meet: digital measurement technology (pupillary distance and fitting heights) built by someone who knows the dispensing counter from the inside.