21 Lens Products & Materials Practice Questions & Answers

1. 1. A patient wearing a +6.00 D sphere prescription says her eyewear feels heavy on the nose after a few hours. Considering only the physical properties of the generic lens materials, which one has the lowest specific gravity and therefore yields the lightest lens for a given volume?

   • A.CR-39 plastic (specific gravity about 1.32)
   • B.Crown glass (specific gravity about 2.54)
   • C.Polycarbonate (specific gravity about 1.20)
   • D.Trivex, a urethane-based material (specific gravity about 1.11)✓ Answer

   Specific gravity is mass per unit volume, so the material with the lowest value makes the lightest lens of the same size and shape. The urethane-based Trivex material, at about 1.11, is the least dense of the common ophthalmic materials, below polycarbonate (about 1.20), CR-39 (about 1.32) and crown glass (about 2.54). Note that this is a weight question, not a thickness question: a high-index material would be thinner but is denser.

   Source: System for Ophthalmic Dispensing, ophthalmic lens materials (specific gravity of generic lens materials)Report a problem with this question

2. 2. Which generic lens material has the lowest Abbe value and therefore shows the most chromatic aberration, seen as coloured fringing when a high-powered lens is viewed away from its optical centre?

   • A.Polycarbonate (Abbe about 30)✓ Answer
   • B.Crown glass (Abbe about 59)
   • C.Trivex (Abbe about 43 to 45)
   • D.CR-39 plastic (Abbe about 58)

   Abbe value is inversely related to dispersion: a high Abbe number means the material splits white light very little, while a low number means it disperses light strongly. Polycarbonate, at about 30, has the lowest Abbe value of the common materials, so a wearer of a strong prescription in polycarbonate is the most likely to report colour fringes when looking through the periphery of the lens. CR-39 and crown glass, in the high 50s, are the least dispersive.

   Source: System for Ophthalmic Dispensing, ophthalmic lens materials (Abbe value and chromatic dispersion)Report a problem with this question

3. 3. A −9.00 D myope asks for "the lightest lenses possible" and has read that 1.74 high-index plastic is the most advanced material available. Which statement comparing 1.74 high-index plastic with Trivex is accurate?

   • A.The 1.74 lens has the same impact resistance as Trivex and can replace it in safety eyewear
   • B.The 1.74 lens will be thinner at the edge, but its greater density means the finished lens is not necessarily lighter than Trivex✓ Answer
   • C.The 1.74 lens is both thinner and lighter, because refractive index and density are inversely related
   • D.The 1.74 lens has a higher Abbe value than Trivex, so it also shows less colour fringing

   Raising the refractive index lets a lens bend light with flatter, shallower curves, so it is thinner for the same power — but density rises along with index rather than falling, so the thinner lens is not automatically lighter. Trivex is the least dense common material, so in a moderate frame it can weigh as little or less than a 1.74 lens. Abbe also falls as index rises (about 33 at 1.74 versus 43 to 45 for Trivex), and only polycarbonate and Trivex carry the impact performance expected of protective eyewear.

   Source: System for Ophthalmic Dispensing, high-index lens materials (index, density and Abbe relationships)Report a problem with this question

4. 4. Why is an anti-reflective coating considered practically essential on lens materials of refractive index 1.60 and above?

   • A.The anti-reflective coating raises the Abbe value of the material and removes chromatic aberration
   • B.The anti-reflective coating is what makes a high-index lens impact resistant
   • C.High-index materials cannot accept a tint unless an anti-reflective coating is applied first
   • D.Surface reflection rises with index — about 8% of light is lost to reflection at n = 1.50 versus roughly 14% at n = 1.74 — so the coating is needed to restore transmission and cosmetic appearance✓ Answer

   The proportion of light reflected at each lens surface is governed by the refractive index: R = ((n − 1)/(n + 1))², which gives roughly 4% per surface at n = 1.50 and about 7% per surface at n = 1.74. Doubling the reflected light both dims the image and produces the visible ghost reflections that patients notice cosmetically, so an anti-reflective coating is routinely paired with high-index materials. Coatings do not change a material's impact resistance, tintability or Abbe value.

   Source: Ophthalmic Lenses and Dispensing, reflection at lens surfaces and anti-reflective coatingsReport a problem with this question

5. 5. An electrician spends much of the day looking overhead at ceiling junction boxes at arm's length and also reads schematics at a bench. Which multifocal category is designed specifically for this pattern of work?

   • A.A blended, line-free round bifocal
   • B.A general-purpose progressive addition lens
   • C.An occupational double-segment lens, with a second near segment placed at the top of the lens✓ Answer
   • D.A round-segment bifocal with a low segment height

   Occupational double-segment designs add a second near or intermediate segment at the top of the lens so the wearer gets near correction while looking up, which a conventional multifocal cannot provide because all of its added power sits below the distance zone. This is the standard recommendation for trades that work overhead at close range, such as electricians, mechanics and stock clerks; a general-purpose progressive still leaves the upper lens set for distance.

   Source: American Board of Opticianry Basic Certification exam content outline, ophthalmic products (occupational lens designs)Report a problem with this question

6. 6. A patient selects a three-piece rimless mounting in which the lenses are drilled and held by screws. Which lens material is the best choice because it resists stress cracking around the drill holes while also giving high impact resistance?

   • A.Trivex✓ Answer
   • B.Crown glass
   • C.CR-39 plastic
   • D.1.74 high-index plastic

   A drilled mounting concentrates mechanical stress at the holes, so the material must tolerate drilling and repeated tightening without chipping or radial cracking. Trivex combines impact resistance comparable to polycarbonate with better internal stress characteristics, so it is the preferred drill-mount material; CR-39 is brittle enough to crack at the holes, glass is unsuitable and heavy, and very high index plastics are more prone to chipping. Polycarbonate is acceptable but Trivex is the superior choice for drilling.

   Source: System for Ophthalmic Dispensing, rimless and drill-mount lens material selectionReport a problem with this question

7. 7. An adult's prescription reads −4.50 −1.00 x 180 for the right eye and "balance" for the left eye. What does this tell the dispenser about the material recommendation?

   • A.The patient has equal vision in both eyes and any material is equally suitable
   • B.The left lens can be plano crown glass because it carries no prescription
   • C.The word balance means the two lenses must be made in different materials to equalise weight
   • D.The patient is functionally monocular, so polycarbonate or Trivex should be recommended for both eyes to protect the remaining seeing eye✓ Answer

   A lens ordered as "balance" is written for an eye with no useful vision; it exists only to match the appearance and weight of the seeing eye's lens. Because an injury to the one seeing eye would leave the patient without functional vision, the standard of care is to recommend an impact-resistant material — polycarbonate or Trivex — in both eyes, and to document that the recommendation was made. Explaining the safer option to the patient is the dispenser's duty to warn.

   Source: American Board of Opticianry Basic Certification exam content outline, product performance and limitations (duty to warn)Report a problem with this question

8. 8. For a given add power, which bifocal segment style produces the greatest amount of image jump?

   • A.An executive (Franklin) segment
   • B.A curve-top segment
   • C.A round 22 segment✓ Answer
   • D.A flat-top 28 segment

   Image jump is the sudden prismatic displacement created when the eye crosses the top of the segment, and it grows with the distance from the segment top down to the segment's optical centre. In a round segment the optical centre sits at the geometric centre of the circle — 11 mm below the top of a round 22 — which is by far the largest such distance; a flat-top segment places its optical centre about 5 mm below the line, a curve-top falls between the two, and an executive has none at all.

   Source: System for Ophthalmic Dispensing, multifocal segment styles and image jumpReport a problem with this question

9. 9. Which segment style produces essentially no image jump, and what is the optical reason?

   • A.An executive (Franklin) segment, because its optical centre lies on the dividing line itself✓ Answer
   • B.A round 24, because its wide curved top spreads the prismatic change out
   • C.A blended round segment, because the transition zone has no visible line
   • D.A flat-top 28, because its optical centre lies about 5 mm below the segment line

   Image jump equals the add power multiplied by the distance in centimetres from the top of the segment to the segment's optical centre, so it vanishes only when that distance is zero. The executive design runs the segment across the full width of the lens with its optical centre on the dividing line, giving zero jump; that is why it is prescribed when jump must be eliminated, despite its weight and cosmetic bulk. Flat-top segments reduce jump but do not remove it, and blending a round segment hides the line without moving the optical centre.

   Source: System for Ophthalmic Dispensing, multifocal segment styles and image jumpReport a problem with this question

10. 10. A flat-top 28 bifocal is ordered with a +2.50 D add, and the segment optical centre lies 5 mm below the top of the segment. Applying Prentice's rule, how much image jump does the wearer experience as the line of sight crosses the segment top?

    • A.1.25 prism diopters✓ Answer
    • B.0.50 prism diopters
    • C.5.00 prism diopters
    • D.2.50 prism diopters

    Prentice's rule states that prism equals the decentration in centimetres multiplied by the power in diopters. Here the relevant power is the add alone (+2.50 D) and the relevant distance is from the segment top to the segment optical centre, 5 mm, which is 0.5 cm: 0.5 × 2.50 = 1.25 prism diopters. Only the segment contributes to jump, so the distance prescription does not enter this calculation.

    Source: Prentice's rule applied to multifocal image jump (prism = centimeters of decentration × diopters)Report a problem with this question

11. 11. Which statement correctly distinguishes image jump from image displacement in a segmented multifocal?

    • A.Jump depends only on the segment — its add power and the distance from segment top to segment optical centre — while displacement is the total prismatic effect at the reading point, combining the distance lens and the segment✓ Answer
    • B.Jump and displacement are two names for the same prismatic effect
    • C.Jump depends on the distance prescription while displacement depends only on the segment style
    • D.Displacement occurs only in progressive lenses and jump occurs only in trifocals

    Image jump is the abrupt shift seen at the instant the eye crosses the segment line, and it is produced by the segment's own prismatic effect at its top edge, so only the add and the segment geometry matter. Image displacement is the steady prismatic effect actually experienced at the near viewing point, where the prism of the distance lens at that point adds to or subtracts from the segment's prism — which is why a minus distance lens with a base-down effect can partly cancel the segment's base-up prism. Confusing the two leads to choosing the wrong segment style.

    Source: System for Ophthalmic Dispensing, image jump versus image displacement in multifocalsReport a problem with this question

12. 12. A trifocal is ordered as a 7 × 28. What do the two numbers designate, and what intermediate power does such a trifocal conventionally provide?

    • A.A 7 mm seg height above the 180 line and a 28 mm inset, with the intermediate power one quarter of the add
    • B.A 7 mm intermediate zone height and a 28 mm segment width, with the intermediate power conventionally half of the add✓ Answer
    • C.A 7 mm segment width and a 28 mm segment height, with the intermediate power equal to the full add
    • D.A 7 diopter add and a 28 mm reading zone, with the intermediate power two thirds of the add

    Trifocal segments are written as height by width: the first number is the vertical depth of the intermediate band and the second is the overall segment width in millimetres. The intermediate portion is conventionally 50% of the add, since the intermediate working distance sits roughly midway in dioptric terms between distance and near — so a 7 × 28 with a +2.50 add carries about +1.25 D in the intermediate band.

    Source: System for Ophthalmic Dispensing, trifocal segment designations and intermediate powerReport a problem with this question

13. 13. Which set of measurements must be taken to order and fit a progressive addition lens correctly?

    • A.Binocular PD plus the frame's horizontal eyesize
    • B.Near PD only, since the reading zone is what the wearer notices
    • C.Monocular PDs for each eye plus an individually measured fitting-cross height at the centre of each pupil✓ Answer
    • D.Binocular distance PD and a standard segment height used for all frames

    A progressive has a narrow corridor and a fitting cross that must sit in front of the pupil, so each eye has to be located individually: faces are rarely symmetrical, and a binocular PD split in half can place one corridor several millimetres off centre, producing asymmetric swim and blur. The fitting-cross height is likewise measured to the centre of the pupil in the actual adjusted frame, not taken from a standard segment height.

    Source: Ophthalmic Lenses and Dispensing, fitting progressive power lenses (monocular centration and fitting cross height)Report a problem with this question

14. 14. According to Minkwitz's theorem, what is the optical consequence of choosing a short-corridor progressive design instead of a longer-corridor design of the same add?

    • A.Distance vision is degraded but the intermediate and near zones become wider
    • B.Unwanted astigmatism beside the corridor increases about twice as fast as the power rises along it, so the faster power build of a short corridor narrows the usable zones and increases peripheral distortion✓ Answer
    • C.The near zone becomes wider, because the add is reached sooner
    • D.Unwanted astigmatism is eliminated, because the corridor is shorter and therefore steeper

    Minkwitz's theorem describes the fixed geometric relationship on a progressive surface: the rate of change of unwanted cylinder moving sideways from the corridor is roughly twice the rate of power change along the corridor. Compressing the same add into a shorter corridor therefore steepens the power gradient and doubles that penalty at the sides, giving narrower clear zones and more noticeable swim. Short corridors are chosen when a shallow frame forces it, accepting that trade-off.

    Source: Ophthalmic Lenses and Dispensing, progressive power lens surface design (Minkwitz's theorem)Report a problem with this question

15. 15. A first-time progressive wearer says distance is clear but she has to tip her chin up and hunt for the reading area, which feels too low. Verification shows the fitting cross sitting several millimetres below the centre of each pupil. What is the best remedy?

    • A.Order a longer corridor at the same fitting height
    • B.Increase the add power by +0.50 D
    • C.Change to a softer peripheral design at the same height in the same frame
    • D.Refit so the fitting cross sits at the centre of the pupil, which raises the whole design — corridor and near zone included — into the wearer's normal line of sight✓ Answer

    The fitting cross fixes the vertical position of the entire progressive design, so setting it below the pupil pushes the corridor and near zone down with it, and the wearer must drop her eyes further or raise her chin to reach the add. Restoring the cross to pupil centre — by adjusting the pads or temples if the lens can be raised in the frame, otherwise by remaking at the correct height — puts the reading zone back where the eyes naturally travel. Lengthening the corridor or increasing the add would make the near zone even harder to reach.

    Source: Ophthalmic Lenses and Dispensing, progressive lens non-adaptation troubleshooting (fitting cross height)Report a problem with this question

16. 16. Why must the vertical depth of the frame (the B measurement) be checked before a progressive lens is ordered?

    • A.The frame must be deep enough to accept the design's minimum fitting height; if it is too shallow, the near zone is edged away and little usable reading area remains✓ Answer
    • B.The B measurement determines the wearer's monocular PD
    • C.The B measurement determines how much unwanted astigmatism the surface will contain
    • D.The laboratory uses the B measurement to set the add power

    Every progressive design specifies a minimum fitting height measured from the lowest point of the lens up to the fitting cross, because the corridor and the near zone occupy the lens below that point. If the frame's vertical depth cannot accommodate the fitting cross plus that corridor, the reading portion is literally cut off during edging and the wearer is left with distance and a fragment of intermediate. This is why shallow frames require a short-corridor design or a different frame.

    Source: Ophthalmic Lenses and Dispensing, progressive lens fitting height and frame selectionReport a problem with this question

17. 17. Why is an aspheric design commonly specified for a high plus prescription such as +7.00 D?

    • A.The surface flattens progressively from centre to edge, allowing a flatter base curve with less centre thickness, less bulge and weight, and less magnification, while controlling the marginal astigmatism a flat spherical curve would introduce✓ Answer
    • B.It increases magnification, which makes the wearer's eyes look larger and more natural
    • C.It makes centration less critical, so the lens can be worn well away from its optical centre
    • D.It removes chromatic aberration, because flattening the surface raises the Abbe value of the material

    A spherical high plus lens needs a steep, bulging front curve; simply flattening that curve would create severe oblique (marginal) astigmatism off axis. An aspheric surface changes curvature progressively away from the optical centre, which keeps peripheral vision clear on a flatter form and simultaneously reduces plate height, thickness, weight and the magnified look of the eyes. The trade-off is that aspheric lenses are far more sensitive to centration, vertex distance and pantoscopic tilt, so fitting must be precise.

    Source: System for Ophthalmic Dispensing, aspheric lens design and marginal astigmatismReport a problem with this question

18. 18. A prescription of −18.00 D sphere is ordered. Which lens form is intended for powers this extreme?

    • A.An executive-style lens so the power is spread across the whole blank
    • B.A lenticular (myodisc) form, with a small central optical aperture surrounded by a non-prescriptive carrier to limit edge thickness and weight✓ Answer
    • C.A blended round segment form
    • D.A single-vision spherical lens on the steepest available base curve

    In very high minus powers the edge thickness grows with the square of the lens diameter, so a full-aperture lens becomes unacceptably thick and heavy at the rim. A lenticular or myodisc form confines the prescription to a small central bowl and surrounds it with a thin carrier that has no power, sharply reducing weight and edge thickness at the cost of a restricted field and a visible dividing line. The same principle in plus form is used for high hyperopic and aphakic corrections.

    Source: System for Ophthalmic Dispensing, lenticular and myodisc lens forms for extreme powersReport a problem with this question

19. 19. A patient reports that his photochromic lenses barely darken while he is driving, although they darken normally when he walks outdoors. What explains this?

    • A.Photochromic lenses darken only when the temperature is above body temperature, and the car is cooler
    • B.Photochromic lenses respond to infrared, which glass transmits freely
    • C.The lenses are defective and must be remade under warranty
    • D.Classic photochromic lenses are triggered by ultraviolet light, and the laminated windshield absorbs most of the ultraviolet that would activate them✓ Answer

    The photochromic reaction in conventional designs is driven by ultraviolet radiation, not by visible brightness or heat. A laminated automobile windshield blocks the great majority of ultraviolet, so the lenses receive little of the energy that darkens them even though the cabin looks bright. Patients who want sun protection while driving should be counselled toward a dedicated tinted or polarized sunwear pair, or a design that also responds to visible light.

    Source: System for Ophthalmic Dispensing, photochromic lens activation by ultraviolet radiationReport a problem with this question

20. 20. How does a polarized lens reduce glare from a wet road or a lake surface, and what limitation should the wearer be warned about?

    • A.An embedded stretched film blocks the horizontally polarized light reflected from horizontal surfaces; the wearer should be warned that liquid-crystal and light-emitting displays may dim or black out at certain head angles✓ Answer
    • B.A gradient tint absorbs the upper field; the wearer should be warned that it provides no ultraviolet protection at all
    • C.The lens darkens automatically where glare strikes it; the wearer should be warned that it will not work in cold weather
    • D.A mirror layer on the back surface reflects the glare away; the wearer should be warned that it wears off within a year

    Light reflected from a horizontal surface such as water, snow or a road becomes largely polarized in the horizontal plane, and a polarized lens is built as a sandwich with a stretched film whose molecules absorb that horizontal component while transmitting the vertical one. The same selectivity is why polarized lenses interact with liquid-crystal instrument panels, phone screens and cockpit displays, which are themselves polarized and can wash out or go black at particular viewing angles — a genuine caution for pilots and heavy-equipment operators.

    Source: System for Ophthalmic Dispensing, polarized lenses and the orientation of the polarizing filterReport a problem with this question

21. 21. A patient asks for a very dark fashion tint in a lens material that has no inherent ultraviolet absorption, and wants to skip any ultraviolet treatment to save money. Why should the dispenser advise against this?

    • A.Ultraviolet treatment is what keeps the tint from fading in sunlight
    • B.A dark tint without ultraviolet treatment will cause the lens to fail its impact resistance
    • C.The dark tint makes the pupil dilate while ultraviolet still passes through, so more ultraviolet reaches the inside of the eye than with no sunglass at all✓ Answer
    • D.A dark tint cannot be applied to any lens unless ultraviolet protection is applied first

    Visible-light density and ultraviolet absorption are independent properties. A dark tint reduces visible brightness, which relaxes the pupillary light reflex and opens the pupil, but if the substrate transmits ultraviolet that wider pupil admits more ultraviolet to the crystalline lens and retina than an unshaded eye would. Materials such as polycarbonate, Trivex and most high-index plastics absorb ultraviolet inherently, whereas CR-39 and crown glass need an added ultraviolet absorber to reach the protection level a sunglass should provide.

    Source: System for Ophthalmic Dispensing, absorptive lenses, tint density and ultraviolet protectionReport 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 →