20 Instrumentation & Observation Practice Questions & Answers

1. 1. A keratometer measures the size of the reflected image of its mires. Which part of the eye does that reflection come from?

   • A.The posterior corneal surface, where most of the eye's refraction occurs
   • B.The anterior surface of the crystalline lens, lying just behind the iris
   • C.The central zone of the anterior corneal surface, about 3 mm across✓ Answer
   • D.The entire cornea, from one limbal edge to the other, in a single reading

   The keratometer projects its mires onto the tear film covering the cornea and computes curvature from the size of the reflected image, so it samples only a small central zone of roughly 3 mm on the front surface. Peripheral corneal shape and the back surface are never assessed, which is why corneal topography is added whenever the shape of the whole cornea matters to the fit.

   Source: Keratometry principles — StatPearls, "Keratometer"; Bennett & Henry, Clinical Manual of Contact Lenses, keratometry chapterReport a problem with this question

2. 2. A keratometer gives a corneal radius of 7.50 mm in one meridian. Using the standard keratometric constant 337.5, what is that meridian's power in diopters?

   • A.43.25 D
   • B.47.50 D
   • C.45.00 D✓ Answer
   • D.42.00 D

   Dioptric power equals 337.5 divided by the radius in millimetres, so 337.5 / 7.50 = 45.00 D. The constant already contains the fictitious refractive index of 1.3375 that the keratometer is built on, because (1.3375 − 1) × 1000 = 337.5; dividing by the index a second time is a frequent candidate error.

   Source: Keratometric constant and the 1.3375 index — StatPearls, "Keratometer"; Borish's Clinical Refraction, keratometryReport a problem with this question

3. 3. Keratometry on the right eye reads 42.50 D in the horizontal meridian and 44.50 D in the vertical meridian. How is this recorded and what does the difference represent?

   • A.44.50 @ 090 / 42.50 @ 180, with 2.00 D of against-the-rule corneal astigmatism
   • B.42.50 @ 180 / 44.50 @ 090, with 2.00 D of with-the-rule corneal astigmatism✓ Answer
   • C.44.50 @ 180 / 42.50 @ 090, with 2.00 D of against-the-rule refractive astigmatism
   • D.42.50 @ 090 / 44.50 @ 180, with 2.00 D of with-the-rule refractive astigmatism

   The flatter meridian is always written first, so the entry begins 42.50 @ 180. The 2.00 D difference between the two principal meridians is corneal astigmatism only, because the keratometer reads the front surface of the cornea and not the eye's total refraction; since the steeper meridian lies near 090 the astigmatism is with-the-rule.

   Source: Recording keratometry and corneal astigmatism — Bennett & Henry, Clinical Manual of Contact Lenses, keratometry chapterReport a problem with this question

4. 4. What is the correct way to focus a keratometer eyepiece before taking readings, and why does that step matter?

   • A.Leave it where the previous examiner set it, so that the instrument's doubling stays calibrated
   • B.Turn it fully counterclockwise and then back until the reticle sharpens, so accommodation is relaxed✓ Answer
   • C.Turn it fully clockwise and then back until the mires soften, so the tear film is given time to settle
   • D.Turn it while viewing the patient's cornea, so that the reticle and the mires focus together

   The eyepiece is focused against a white card held behind the mires: it is turned fully counterclockwise into plus and then slowly back until the reticle crosshair first appears sharp, which relaxes the examiner's accommodation. If it is left set for another observer, the examiner accommodates to clear the mires and every reading shifts, which is among the most frequently cited sources of keratometry error.

   Source: Keratometer eyepiece focusing procedure — Bennett & Henry, Clinical Manual of Contact Lenses; StatPearls, "Keratometer"Report a problem with this question

5. 5. During keratometry the mires look wavy and broken and cannot be brought into a clean overlap in either meridian. What does this finding indicate?

   • A.An irregular anterior corneal surface, from keratoconus, warpage or a poor tear film✓ Answer
   • B.A recording error that is corrected by writing the flat meridian first in the chart
   • C.A lens power so high that an auxiliary lens must be added over the objective
   • D.A regular astigmatism so high that it exceeds the drum range of the instrument

   Mire quality is a qualitative finding rather than a number: a reflected image can only be as regular as the surface reflecting it. An unstable or deficient tear film, epithelial irregularity, corneal warpage from rigid lens wear, or an ectasia such as keratoconus all break the mires up. The correct response is to have the patient blink, re-wet the eye and, if the distortion persists, obtain topography, not to force a reading.

   Source: Distorted keratometry mires and irregular astigmatism — StatPearls, "Keratoconus"; Bennett & Henry, Clinical Manual of Contact LensesReport a problem with this question

6. 6. A cornea is steeper than the keratometer's standard drum range, so an auxiliary lens is placed over the objective. Which lens is used and how is the drum reading adjusted?

   • A.A −1.00 D lens is used, and roughly 6.00 D is subtracted from the reading
   • B.A +1.25 D lens is used, and roughly 9.00 D is subtracted from the reading
   • C.A +1.25 D lens is used, and roughly 9.00 D is added to the drum reading✓ Answer
   • D.A −1.00 D lens is used, and roughly 6.00 D is added to the drum reading

   A +1.25 D lens taped over the objective extends the range upward for steep corneas; the drum then under-reads, so approximately 9.00 D is added to what it shows. A −1.00 D lens extends the range downward for flat corneas and approximately 6.00 D is subtracted. The manufacturer's conversion table or nomogram gives the exact converted value, and the added or subtracted figures are working approximations.

   Source: Extending keratometer range with auxiliary lenses — Bennett & Henry, Clinical Manual of Contact Lenses, keratometry chapterReport a problem with this question

7. 7. How does a one-position keratometer differ from a two-position instrument in obtaining the two principal meridians?

   • A.The one-position instrument varies the object size and must be rotated for the second meridian
   • B.The two-position instrument keeps the object size fixed and reads both meridians at once
   • C.The two-position instrument varies the doubling and reads both meridians without rotating
   • D.The one-position instrument varies the doubling and reads both meridians without rotating✓ Answer

   The one-position (Bausch & Lomb type) keratometer holds the object size fixed and varies the doubling, so both principal meridians are read with the instrument in a single position. The two-position (Javal–Schiötz type) holds the doubling fixed and varies the object size, so it must be rotated onto the second meridian. Several widely used study guides state this pairing backwards.

   Source: One-position versus two-position keratometers — StatPearls, "Keratometer"; Borish's Clinical Refraction, keratometryReport a problem with this question

8. 8. How is a keratometer checked for accurate calibration in the office?

   • A.Read the mires off a white card with the eyepiece turned fully to its plus stop
   • B.Read one patient's cornea three times in a row and average the drum values obtained
   • C.Read a rigid lens of known base curve set concave side up on the instrument headrest
   • D.Read a set of polished steel spheres of known radius and compare against their stated values✓ Answer

   Calibration is verified against a physical standard rather than against an eye. After the eyepiece has been focused, polished steel spheres of certified radius are read, and any consistent departure from the stated radius means the instrument needs adjustment or servicing. A cornea cannot serve as a standard because its true curvature is unknown and its tear film changes between blinks.

   Source: Keratometer calibration with steel test spheres — Bennett & Henry, Clinical Manual of Contact Lenses; StatPearls, "Keratometer"Report a problem with this question

9. 9. Which slit lamp illumination setting is used routinely to evaluate the fluorescein pattern of a rigid lens on the eye?

   • A.A parallelepiped a few millimetres wide, set at a moderate illumination angle✓ Answer
   • B.A conical pinhole beam in a darkened room, viewed almost straight on
   • C.A specular reflection setting with matched angles, at the highest magnification
   • D.An optic section under one millimetre wide, set at a wide illumination angle

   A parallelepiped — a beam a few millimetres wide set at roughly 45 degrees — lights a three-dimensional block of tissue broad enough to survey the whole lens-cornea interface at once while still giving depth, which is exactly what fluorescein pattern reading requires. An optic section is too narrow for pattern work, a conical beam is meant for anterior chamber cells and flare, and specular reflection examines surface quality rather than tear layer thickness.

   Source: Slit lamp illumination techniques in contact lens practice — Efron, Contact Lens Complications, slit lamp biomicroscopy chapterReport a problem with this question

10. 10. The beam is aimed at the iris while the microscope stays focused on the contact lens in front of it. Which findings does this technique show best?

    • A.The lipid layer of the tear film and the endothelial cell mosaic at high power
    • B.The stromal depth of a corneal scar and the relative thickness of the cornea
    • C.Suspended cells and flare in the aqueous of the anterior chamber in a dark room
    • D.Lens surface deposits, epithelial microcysts and fine vessels in silhouette✓ Answer

    Retroillumination uses light returning from the iris or fundus as a backlight, so anything semi-transparent in front of it — surface deposits, epithelial microcysts, vacuoles, fine new vessels, faint scars — is seen in silhouette or as a shadow. Lighting those same changes directly from the front washes them out against the bright reflex, which is why direct techniques miss them.

    Source: Retroillumination — Efron, Contact Lens Complications, slit lamp biomicroscopy chapterReport a problem with this question

11. 11. The slit beam is decentered onto the limbus while the examiner watches the central cornea. What makes a central abnormality visible with this technique?

    • A.Light travels through the cornea by total internal reflection and is scattered by any opacity✓ Answer
    • B.Light is doubled by a prism so that the two images of an opacity can be aligned
    • C.Light is absorbed by the limbal vessels, darkening the field and raising contrast
    • D.Light reflected off the iris backlights the cornea and shows opacities in silhouette

    In sclerotic scatter the beam entering at the limbus is trapped inside the cornea by total internal reflection and is piped around it. A clear cornea stays dark, but edema, haze, infiltrates or a foreign body scatter the trapped light and glow against the dark pupil. It is therefore a rapid screen for central corneal edema in a lens wearer.

    Source: Sclerotic scatter — Efron, Contact Lens Complications, slit lamp biomicroscopy chapterReport a problem with this question

12. 12. Why is the upper lid everted at a contact lens follow-up visit?

    • A.To reach the lacrimal gland, where a Schirmer strip is placed to sample basal tears
    • B.To view the upper tarsal conjunctiva, where giant papillary conjunctivitis begins✓ Answer
    • C.To expose the meibomian orifices, which are counted to grade the degree of edema
    • D.To view the superior limbus, where hypoxic new vessels first cross onto the cornea

    The papillae of giant papillary conjunctivitis grow on the tarsal conjunctiva of the upper lid, a surface that stays completely hidden until the lid is rolled back, so a wearer can have marked disease with an otherwise unremarkable anterior segment. Eversion is performed with the patient looking down while the examiner grasps the lashes and rolls the lid over a cotton applicator or a fingertip.

    Source: Lid eversion and papillary conjunctivitis grading — Efron, Contact Lens Complications, giant papillary conjunctivitis chapterReport a problem with this question

13. 13. What is a Burton lamp used for in contact lens practice?

    • A.It gives a narrow slit of light for locating the depth of an opacity in the stroma
    • B.It gives white light at a fixed working distance for measuring rigid lens diameter
    • C.It gives a calibrated target whose reflected image yields the radius of a surface
    • D.It gives broad cobalt blue illumination with low magnification for a fluorescein pattern✓ Answer

    The Burton lamp is a hand-held ultraviolet and cobalt blue fluorescent lamp fitted with a low-power magnifier. It floods the eye with an even blue field so a rigid lens fluorescein pattern and gross corneal staining can be judged quickly, but it offers no stereoscopic view, no variable beam and no optic section, so detailed corneal assessment still belongs to the slit lamp.

    Source: Burton lamp and cobalt blue fluorescein evaluation — Bennett & Henry, Clinical Manual of Contact Lenses, rigid lens fitting evaluationReport a problem with this question

14. 14. Corneal staining must be assessed with a silicone hydrogel lens still on the eye. Which dye is correct and why?

    • A.Standard sodium fluorescein, followed by rinsing the lens with sterile saline
    • B.Standard sodium fluorescein, because a silicone hydrogel surface repels the dye
    • C.High-molecular-weight fluorescein, because it will not enter the lens matrix✓ Answer
    • D.Rose bengal dye, because it stains only the lens surface and not the matrix

    Sodium fluorescein is a small molecule that soaks into a hydrogel or silicone hydrogel matrix and stains it permanently, and rinsing does not remove it. High-molecular-weight fluorescein is too large to enter the matrix, so it can be used with the lens in place. The alternative is to remove the lens, stain the cornea, and wait roughly half an hour to an hour before reinserting it.

    Source: Use of high-molecular-weight fluorescein with soft lenses — Efron, Contact Lens Complications, corneal staining chapterReport a problem with this question

15. 15. A rigid lens shows a bright pool of fluorescein centrally, a dark ring of contact in the mid-periphery, and almost no movement on blink. How is this fit described?

    • A.The base curve aligns with the cornea, so no parameter change is needed
    • B.The base curve is correct but the overall diameter is far too small
    • C.The base curve is flatter than the cornea, so the lens rests on the apex
    • D.The base curve is steeper than the cornea, so the lens vaults the apex✓ Answer

    Fluorescein brightness follows the thickness of the tear layer under the lens, so bright green pooling marks clearance and dark areas mark bearing. Central pooling with mid-peripheral touch and minimal movement is the classic steep, apical-clearance fit. A flat lens gives the reverse picture: a dark central bearing area, heavy peripheral pooling with excess edge lift, and too much movement.

    Source: Interpretation of rigid lens fluorescein patterns — Bennett & Henry, Clinical Manual of Contact Lenses, rigid lens fittingReport a problem with this question

16. 16. A radiuscope is used on a rigid lens. Which parameter does it give, and how is the value derived?

    • A.The base curve, from the travel between the surface image and the aerial image✓ Answer
    • B.The base curve, from the single dial reading where the surface image is sharpest
    • C.The back vertex power, from the travel between the two focus positions
    • D.The center thickness, from the dial reading taken at the aerial image

    The radiuscope applies the Drysdale principle: the microscope is focused first on the target image formed at the lens surface and then on the aerial image at the center of curvature, and the distance the instrument travels between those two positions equals the radius of curvature of the back optic zone, which is the base curve. A single focus position yields nothing, and power and thickness are measured by other instruments.

    Source: Radiuscope (optical spherometer) and the Drysdale principle — Bennett & Henry, Clinical Manual of Contact Lenses, rigid lens verificationReport a problem with this question

17. 17. A rigid lens is being verified for back vertex power on a lensometer. How is it placed?

    • A.Concave side down inside a saline wet cell set against the standard stop
    • B.Concave side up on the standard spectacle stop, held by the lens table
    • C.Concave side down on a reduced contact lens aperture over the stop✓ Answer
    • D.Concave side up on a reduced contact lens aperture, wetted with saline

    Back vertex power is measured from the back surface, so the lens is set with its concave side against the stop. A reduced contact lens aperture is fitted because the standard spectacle stop is far wider than the lens and would sample outside the optic zone, giving an unreliable reading. A wet cell belongs to soft lens verification rather than to routine rigid lens power checking.

    Source: Verifying contact lens power on the lensometer — Bennett & Henry, Clinical Manual of Contact Lenses, lens verificationReport a problem with this question

18. 18. A soft lens power read in a saline wet cell must be corrected before it is compared with the ordered power. Why?

    • A.The saline around the lens reduces the index difference at its surfaces✓ Answer
    • B.The wet cell introduces prism that shifts the mires off the optical center
    • C.The saline evaporates during the reading and steepens the front surface
    • D.The wet cell magnifies the target, so the axis reads rotated from its true value

    The power of a lens depends on the difference in refractive index between the lens material and the medium surrounding it. In air that difference is large, but in saline it is much smaller, so a wet-cell reading is lower than the lens's power in air and the correction factor supplied for the cell must be applied. Blotting the lens and reading it quickly in air avoids the correction but risks a dehydration error.

    Source: Soft lens power verification by wet cell — Bennett & Henry, Clinical Manual of Contact Lenses, soft lens verificationReport a problem with this question

19. 19. Which instrument gives the overall diameter of a rigid lens, and which gives its center thickness?

    • A.A V-channel gauge gives the diameter; a dial thickness gauge gives the thickness✓ Answer
    • B.A radiuscope gives the diameter; a measuring magnifier gives the thickness
    • C.A dial thickness gauge gives the diameter; a V-channel gauge gives the thickness
    • D.A lensometer gives the diameter; a radiuscope gives the thickness reading

    The lens is slid concave side down along a tapered V-channel until it wedges, and the scale at that point reads overall diameter; a dial thickness gauge reads center thickness, which matters clinically because thickness drives oxygen transmissibility (Dk/t). Optic zone diameter is read separately on a measuring magnifier with a reticle, and the radiuscope measures base curve only.

    Source: Rigid lens verification: diameter, optic zone and thickness — Bennett & Henry, Clinical Manual of Contact Lenses, lens verificationReport a problem with this question

20. 20. Between patients, how should the instrument surfaces and the diagnostic lenses used in a fitting be handled?

    • A.Leave the rests untouched when no visible contact occurred and store diagnostic lenses dry
    • B.Clean the rests at the end of the clinic day and soak diagnostic lenses in saline overnight
    • C.Wipe the rests with a dry tissue and rinse each diagnostic lens under running tap water
    • D.Disinfect the chin and forehead rests and any tissue-contacting tip, and disinfect each diagnostic lens✓ Answer

    Every surface that touches a patient — the chin rest, the forehead rest, hand-held lamp housings and any tip that contacts tissue — is disinfected between patients, and a diagnostic lens is either disinfected by an approved method or discarded after a single use. Tap water is never used on lenses because it can carry Acanthamoeba, and dry storage of a soiled diagnostic lens neither cleans nor disinfects it.

    Source: CDC guidance on disinfection of patient-care devices and equipment; FDA classification of contact lenses and lens care products as medical devicesReport 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 →