21 Ophthalmic Optics Practice Questions & Answers

1. 1. A material has a refractive index of 1.586. What does this value tell you about light travelling inside that material?

   • A.Light travels 1.586 times faster inside the material than in a vacuum
   • B.Light travels 1.586 times slower inside the material than in a vacuum✓ Answer
   • C.Light is dispersed into 1.586 separate wavelengths inside the material
   • D.Light is absorbed by 1.586 percent at each surface of the material

   Refractive index is defined as the speed of light in a vacuum divided by the speed of light in the material, so an index of 1.586 means light is slowed to 1/1.586 of its vacuum speed. Because the material slows light more, it also bends light more for a given surface curve, which is why higher-index materials produce the same lens power with flatter, thinner surfaces.

   Source: Definition of index of refraction, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

2. 2. A thin spectacle lens has a focal length of 25 cm. What is its dioptric power?

   • A.0.25 D
   • B.2.50 D
   • C.25.00 D
   • D.4.00 D✓ Answer

   Dioptric power is the reciprocal of the focal length expressed in metres, so 25 cm must first be converted to 0.25 m and then D = 1 / 0.25 = 4.00 D. Leaving the focal length in centimetres produces the classic distractor of 0.25 D, which is why the conversion to metres must be done before taking the reciprocal.

   Source: Relationship D = 1/f (f in metres), standard ophthalmic optics; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

3. 3. Which statement correctly describes how a minus (concave) lens acts on light and the image it forms?

   • A.It converges light and forms a real, inverted image
   • B.It converges light and forms a virtual, inverted image
   • C.It diverges light and forms a virtual, erect, minified image✓ Answer
   • D.It diverges light and forms a real, inverted image beyond the lens

   A concave lens is thinnest at its centre and thickest at its edge, so it can be modelled as two prisms with their apices together at the optical centre; since light bends toward the base, the rays are spread apart and diverge. Diverging rays never actually cross, so the image can only be located by extending them backwards, which produces a virtual, erect and minified image.

   Source: Minus lens form and virtual image conventions, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

4. 4. Light travelling inside a dense medium strikes the boundary with air at an angle greater than the critical angle. What happens to that light?

   • A.It passes into the air bent toward the normal
   • B.It is entirely reflected back into the denser medium (total internal reflection)✓ Answer
   • C.It passes into the air bent away from the normal
   • D.It is split into its component wavelengths as it enters the air

   The critical angle is the angle of incidence inside the denser medium at which the refracted ray would emerge along the surface itself, and beyond that angle no refracted ray can exist. All the light is therefore reflected back inside the medium, a condition called total internal reflection, which is also why the edges of high-index lenses can show bright internal reflections.

   Source: Critical angle and total internal reflection, standard ophthalmic optics texts; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

5. 5. A patient reports coloured fringes around objects seen through the outer portion of new lenses. Which lens property is the direct measure of this effect?

   • A.The specific gravity, where a higher number means more chromatic dispersion
   • B.The Abbe value, where a lower number means more chromatic dispersion✓ Answer
   • C.The Abbe value, where a lower number means less chromatic dispersion
   • D.The base curve, where a flatter curve means more chromatic dispersion

   Abbe value, also called constringence, quantifies how much a material splits white light into its component wavelengths, and the scale runs inversely so that a low Abbe number indicates high dispersion. Polycarbonate sits near the bottom of the common materials at roughly 30, which is why colour fringing is most often reported at the periphery of polycarbonate lenses, while crown glass and CR-39 near 58 to 59 show it least.

   Source: Abbe value (constringence) and chromatic aberration, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

6. 6. A patient looks through a point 6 mm above the optical centre of a +5.00 D lens. What prism is induced at that point?

   • A.3.00 prism dioptres base up
   • B.0.30 prism dioptres base down
   • C.30.00 prism dioptres base down
   • D.3.00 prism dioptres base down✓ Answer

   Prentice's rule states that induced prism equals the decentration in centimetres multiplied by the lens power, so 6 mm becomes 0.6 cm and 0.6 x 5.00 = 3.00 prism dioptres. A plus lens behaves like two prisms base to base at the optical centre, so a point above the centre lies in the base-down half of the lens and the induced prism is base down.

   Source: Prentice's rule and base-direction convention, National Academy of Opticianry approved course on Prentice's Rule; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

7. 7. A patient views through a point 4 mm below the optical centre of a -3.50 D lens. What prism is induced?

   • A.0.88 prism dioptres base up
   • B.1.40 prism dioptres base up
   • C.14.00 prism dioptres base down
   • D.1.40 prism dioptres base down✓ Answer

   Converting 4 mm to 0.4 cm and applying Prentice's rule gives 0.4 x 3.50 = 1.40 prism dioptres. A minus lens acts like two prisms apex to apex at the optical centre, so its base directions are the reverse of a plus lens: looking below the optical centre of a minus lens yields base down, and looking above it yields base up.

   Source: Prentice's rule and base-direction convention for minus lenses, National Academy of Opticianry approved course on Prentice's Rule; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

8. 8. What is the defining optical property of the optical centre of a spectacle lens?

   • A.It is the geometric centre of the finished lens shape after edging
   • B.It is the point at which the front and back surface powers are equal
   • C.It is the point at which the lens is always at its thickest
   • D.It is the single point on the lens at which no prismatic deviation is produced✓ Answer

   The optical centre is defined as the point where light passes through the lens without being deviated, which follows directly from Prentice's rule since the decentration distance there is zero. It is not necessarily the thickest point, and it coincides with the geometric centre of the edged lens only when no decentration has been ordered.

   Source: Definition of optical centre and Prentice's rule, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

9. 9. A lens has a front surface power of +8.00 D and a back surface power of -5.25 D. What is its approximate (nominal) power?

   • A.+1.38 D
   • B.+13.25 D
   • C.+2.75 D✓ Answer
   • D.-2.75 D

   Nominal or approximate lens power is the algebraic sum of the two surface powers, so +8.00 added to -5.25 gives +2.75 D. This sum ignores lens thickness and index, which is why it is called approximate power and why a lensometer reading of back vertex power will differ slightly from it as thickness and power increase.

   Source: Nominal lens power F = F1 + F2, standard ophthalmic optics; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

10. 10. A convex surface with a radius of curvature of 0.100 m is ground on a material of index 1.60. What is the surface power?

    • A.+6.00 D✓ Answer
    • B.+0.60 D
    • C.+5.30 D
    • D.+16.00 D

    Surface power is calculated as (n - 1) divided by the radius of curvature in metres, so (1.60 - 1) / 0.100 = 0.60 / 0.100 = +6.00 D. The formula shows why a higher-index material reaches the same surface power with a longer radius, meaning a flatter and therefore thinner lens.

    Source: Surface power formula D = (n - 1)/r, standard ophthalmic optics; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

11. 11. A lens clock is calibrated for an index of 1.53 and reads +6.00 D on the front surface of a polycarbonate lens of index 1.586. What is the true surface power?

    • A.Approximately +5.43 D
    • B.Exactly +6.00 D
    • C.Approximately +9.51 D
    • D.Approximately +6.63 D✓ Answer

    A lens clock measures sagittal depth and converts it to power using its own calibration index, so the reading must be rescaled as reading x (n actual - 1) divided by (n calibration - 1), giving 6.00 x 0.586 / 0.53 = 6.63 D. Because polycarbonate has a higher index than the 1.53 calibration, the same physical curve produces more power than the dial indicates.

    Source: Lens measure calibration index correction, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

12. 12. What distinguishes a toric (cylindrical) surface from a spherical surface, and what does each correct?

    • A.A spherical surface has two different curves 90 degrees apart and corrects astigmatism, while a toric surface has one uniform curve and corrects myopia
    • B.A toric surface has curves that differ by 45 degrees and is used only for presbyopia
    • C.A spherical surface has the same curve in every meridian and corrects myopia or hyperopia, while a toric surface has two different principal curves 90 degrees apart and corrects regular astigmatism✓ Answer
    • D.Both surfaces have identical curves in every meridian, but only the toric one is polished

    A spherical surface has a single radius in every meridian, so it brings light to one focal point and corrects only the spherical component of a refractive error. A toric surface has two principal meridians of unequal curvature exactly 90 degrees apart, producing two focal lines rather than one point, which is precisely what is needed to neutralise regular astigmatism.

    Source: Spherical and toric surfaces, principal meridians, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

13. 13. For the prescription -2.00 -2.00 x 135, what is the total power in the 90 degree meridian?

    • A.-3.00 D✓ Answer
    • B.-2.00 D
    • C.-2.50 D
    • D.-4.00 D

    Power in any meridian equals the sphere plus the fraction of the cylinder given by the sine squared of the angle between the axis and that meridian, and the practical shortcut assigns 0 percent at 0 degrees, 25 percent at 30, 50 percent at 45, 75 percent at 60 and 100 percent at 90. The 90 meridian sits 45 degrees from an axis of 135, so half of -2.00 is -1.00, which added to the -2.00 sphere gives -3.00 D.

    Source: Power in oblique meridians, sine-squared rule, National Academy of Opticianry approved course on Prentice's Rule; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

14. 14. A prism deviates a ray of light 3 cm on a screen placed 1 m away. What is its power, and where does the image appear relative to the prism?

    • A.0.3 prism dioptres, with the image displaced toward the apex
    • B.3 prism dioptres, with the image displaced toward the base
    • C.3 prism dioptres, with the image displaced toward the apex✓ Answer
    • D.30 prism dioptres, with the image displaced toward the base

    One prism dioptre is defined as one centimetre of deviation of a light ray measured at a distance of one metre, so 3 cm at 1 m is exactly 3 prism dioptres. The ray itself bends toward the base of the prism, but the eye projects the image back along the incoming direction, so the image is always displaced toward the apex.

    Source: Definition of the prism dioptre and base/apex deviation conventions, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

15. 15. A lens carries 3 prism dioptres base out combined with 4 prism dioptres base up. What is the resultant prism power?

    • A.7 prism dioptres
    • B.5 prism dioptres✓ Answer
    • C.1 prism dioptre
    • D.3.5 prism dioptres

    Horizontal and vertical prism components act at right angles to one another, so they combine as the two legs of a right triangle and the resultant is the square root of the sum of their squares. Here the square root of (3 squared plus 4 squared) equals the square root of 25, which is 5 prism dioptres; simply adding the two values would be the common error.

    Source: Resolution and compounding of prism components, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

16. 16. Which aberration is the reason that corrected-curve (best-form) lens series specify a particular base curve for each power range?

    • A.Chromatic aberration, which depends only on the material's Abbe value
    • B.Oblique (marginal) astigmatism, which blurs vision when the eye rotates away from the optical axis✓ Answer
    • C.Total internal reflection at the lens edge
    • D.Diffraction produced by the edge of the lens bevel

    When the eye rotates behind a spectacle lens it looks through the periphery obliquely, and a single-vision spherical surface then forms two focal lines instead of a point, an error called oblique or marginal astigmatism. Corrected-curve series choose a base curve for each power range that cancels this off-axis error, which is also why an aspheric design can flatten the base curve while still controlling it.

    Source: Oblique astigmatism and corrected-curve (best-form) lens design, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

17. 17. Which description correctly matches an aberration to its optical cause?

    • A.Distortion occurs because different wavelengths of light focus at different distances behind the lens
    • B.Coma occurs because the lens material has an unusually high Abbe value
    • C.Curvature of field occurs because the two principal meridians of a toric surface have unequal power
    • D.Spherical aberration occurs because rays through the periphery of a spherical surface focus at a different point than rays through the centre✓ Answer

    Spherical aberration arises from the geometry of a spherical surface itself: peripheral zones bend light more strongly than the central zone, so peripheral rays cross the axis short of the paraxial focus. The other three statements misattribute causes, since wavelength-dependent focus is chromatic aberration, unequal principal meridians describe astigmatism of the lens surface rather than curvature of field, and a high Abbe value reduces rather than creates aberration.

    Source: Named monochromatic lens aberrations, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

18. 18. In an uncorrected myopic eye viewing a distant object, where does the image of that object form?

    • A.Behind the retina, so a plus lens is needed to move the focus forward onto it
    • B.Exactly on the retina, but only in one meridian
    • C.In front of the retina, so a minus lens is needed to move the focus back onto it✓ Answer
    • D.On the retina, with the blur caused solely by loss of accommodation

    Myopia means the eye's optical system is too powerful for its axial length, so parallel light from a distant object is brought to focus in front of the retina. A diverging minus lens reduces the incoming vergence so that the eye's own focus falls further back and lands on the retina.

    Source: Refractive errors described by focal point location, Borish's Clinical Refraction; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

19. 19. Which statement about presbyopia is optically correct?

    • A.It is corrected by adding minus power because the near focal point falls behind the retina
    • B.It is an age-related loss of accommodative amplitude rather than a misplacement of the distance focal point, so it is corrected by adding plus power for near✓ Answer
    • C.It is a refractive error in which distant light focuses behind the retina from birth
    • D.It is caused by the cornea developing two unequal principal meridians with age

    Unlike myopia, hyperopia and astigmatism, presbyopia does not change where distant light focuses; it is the progressive loss of the crystalline lens's ability to increase its power for near viewing. The correction therefore supplies the missing convergence with an add of plus power, which is why an add appears in the prescription rather than a change in the distance sphere.

    Source: Presbyopia and accommodative amplitude, Borish's Clinical Refraction; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

20. 20. A -10.00 D lens is measured at a vertex distance of 12 mm and then refitted so it sits 2 mm closer to the eye. Using the approximation that the dioptric change equals (millimetres of change x power x power) divided by 1000, what power is required at the new position?

    • A.-10.20 D
    • B.-12.00 D
    • C.-8.00 D
    • D.-9.80 D✓ Answer

    The approximation gives (2 x 10 x 10) / 1000 = 0.20 D of change. Moving any lens closer to the eye requires less minus power (or more plus power) to keep the same effective power at the eye, so -10.00 becomes -9.80 D; note that compensation only matters clinically at roughly 4.00 D and above.

    Source: Vertex distance compensation, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport a problem with this question

21. 21. Two lenses of identical dioptric power and identical diameter are made, one in a material of index 1.50 and one in a material of index 1.74. Which statement about the higher-index lens is optically correct?

    • A.It has identical surface curves and thickness, differing only in weight
    • B.It is thinner and also has less surface reflection, because reflection falls as index rises
    • C.It reaches the same power with flatter surface curves, so it is thinner, but it usually has a lower Abbe value and reflects more light at each surface✓ Answer
    • D.It reaches the same power with steeper surface curves, so it is thicker, but it has a higher Abbe value

    Because surface power equals (n - 1) divided by the radius, a higher index reaches the same power with a longer radius, meaning a flatter curve, less sagittal depth and a thinner lens. The trade-offs follow from the same physics: dispersion generally rises as index rises, so Abbe falls, and surface reflection calculated as ((n - 1)/(n + 1)) squared increases, which is why anti-reflective treatment matters most on high-index lenses.

    Source: Index of refraction, surface power, Abbe value and surface reflection relationships, System for Ophthalmic Dispensing; ABO Basic Certification exam content outline, ophthalmic optics domainReport 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 →