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DHA Optometrist study materials
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DHA Optometrist practice questions

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Question 1Ocular Anatomy, Physiology and Optics

Which refractive component of the eye provides the greatest amount of total dioptric power?

  1. The crystalline lens
  2. The vitreous body
  3. The cornea Correct answer.
  4. The anterior chamber

Explanation

The cornea contributes roughly two-thirds of the eye's total refractive power (about 40-43 D), largely because of the air-to-tear film interface. The crystalline lens adds about 15-20 D at rest. The vitreous and anterior chamber are transparent media that do not contribute refractive power.

Question 2Ocular Anatomy, Physiology and Optics

Aqueous humour is produced primarily by which structure?

  1. The corneal endothelium
  2. The ciliary epithelium of the pars plicata Correct answer.
  3. The retinal pigment epithelium
  4. The trabecular meshwork

Explanation

Aqueous humour is secreted by the ciliary epithelium covering the pars plicata of the ciliary body. The corneal endothelium and trabecular meshwork are involved in outflow, not production. The retinal pigment epithelium supports photoreceptors and does not secrete aqueous.

Question 3Ocular Anatomy, Physiology and Optics

Which photoreceptor type is responsible for high-acuity colour vision under photopic conditions?

  1. Rods
  2. Horizontal cells
  3. Cones Correct answer.
  4. Bipolar cells

Explanation

Cones mediate photopic (bright-light) vision and provide high visual acuity and colour discrimination. Rods support scotopic vision and lack colour sensitivity. Bipolar and horizontal cells are second-order neurons that process signals rather than transduce light.

Question 4Ocular Anatomy, Physiology and Optics

The blind spot of the visual field corresponds to which anatomical feature?

  1. The fovea centralis
  2. The macula lutea
  3. The ora serrata
  4. The optic disc Correct answer.

Explanation

The optic disc is the site where retinal ganglion cell axons exit the eye and where retinal vessels enter; it contains no photoreceptors, producing a physiological scotoma. The fovea and macula are regions of highest acuity. The ora serrata is the anterior termination of the retina.

Question 5Ocular Anatomy, Physiology and Optics

Which extraocular muscle is responsible for elevation of the eye when it is in the abducted position?

  1. Medial rectus
  2. Superior oblique
  3. Superior rectus Correct answer.
  4. Inferior rectus

Explanation

The superior rectus is the primary elevator when the eye is abducted, because its axis of action then aligns with the vertical meridian. The superior oblique depresses in adduction. The inferior rectus depresses, and the medial rectus adducts.

Question 6Ocular Anatomy, Physiology and Optics

What is the normal axial length of the adult human eye?

  1. Approximately 20 mm
  2. Approximately 24 mm Correct answer.
  3. Approximately 16 mm
  4. Approximately 32 mm

Explanation

The adult emmetropic eye has an axial length of about 23.5-24 mm. Shorter eyes tend toward hyperopia and longer eyes toward myopia. Values far outside this range are not typical of the adult eye.

Question 7Ocular Anatomy, Physiology and Optics

The corneal endothelium maintains corneal deturgescence primarily through which mechanism?

  1. Continuous keratinisation of the epithelium
  2. Secretion of aqueous humour into the stroma
  3. Active bicarbonate and sodium transport with a leaky tight-junction barrier Correct answer.
  4. Passive diffusion alone with no metabolic activity

Explanation

Endothelial cells actively pump ions, creating an osmotic gradient that draws water from the stroma, while their leaky junctions permit controlled fluid movement. This metabolic pumping keeps the stroma relatively dehydrated and clear. Passive diffusion alone cannot maintain deturgescence, and the endothelium does not secrete aqueous.

Question 8Ocular Anatomy, Physiology and Optics

A patient's far point is 50 cm in front of the eye. What is the magnitude of their myopia?

  1. 2.00 D Correct answer.
  2. 5.00 D
  3. 0.20 D
  4. 0.50 D

Explanation

The far point distance in metres gives the refractive error: 1/0.50 m equals 2.00 D of myopia. The far point is the farthest point that can be seen clearly without accommodation. A 0.50 m far point therefore corresponds to 2.00 D, not 0.50 D.

Question 9Ocular Anatomy, Physiology and Optics

Which layer of the tear film is produced mainly by the meibomian glands?

  1. The middle aqueous layer
  2. The corneal epithelial glycocalyx
  3. The superficial lipid layer Correct answer.
  4. The inner mucin layer

Explanation

Meibomian glands secrete lipids that form the outermost layer of the tear film, retarding evaporation. The aqueous layer comes from the lacrimal and accessory glands, and the mucin layer from conjunctival goblet cells. The glycocalyx is part of the corneal epithelium, not a tear film layer.

Question 10Ocular Anatomy, Physiology and Optics

Which statement best describes the optics of a concave (minus) lens?

  1. It has positive power and is used for hyperopia
  2. It converges incident light and forms a real image
  3. It has no effect on the vergence of light
  4. It diverges incident light and forms a virtual image Correct answer.

Explanation

A concave lens is thinner at the centre, diverges parallel rays, and has negative power; it forms a virtual image on the same side as the object. Convex lenses converge light and have positive power. All refracting lenses alter vergence.

Question 11Ocular Anatomy, Physiology and Optics

Which retinal layer contains the cell bodies of the photoreceptors?

  1. The inner plexiform layer
  2. The nerve fibre layer
  3. The outer nuclear layer Correct answer.
  4. The ganglion cell layer

Explanation

The outer nuclear layer houses the nuclei of rods and cones. The ganglion cell layer contains ganglion cell bodies, the inner plexiform layer contains synapses, and the nerve fibre layer contains ganglion cell axons.

Question 12Ocular Anatomy, Physiology and Optics

What is the primary function of the ciliary muscle during accommodation?

  1. Relaxation tightens the zonules, increasing lens power
  2. Contraction tightens the zonules, flattening the lens
  3. Contraction relaxes the zonules, allowing the lens to become more convex Correct answer.
  4. It directly changes the curvature of the cornea

Explanation

During accommodation the ciliary muscle contracts, moving inward and forward, which slackens the zonular fibres. The elastic lens capsule then allows the lens to assume a more spherical, higher-power shape. Zonular tension flattens the lens when accommodation is relaxed.

Question 13Ocular Anatomy, Physiology and Optics

A ray of light travelling from air into the cornea will behave in which way?

  1. It bends toward the normal because the cornea has a higher refractive index Correct answer.
  2. It continues undeviated regardless of incidence angle
  3. It is totally internally reflected at the anterior surface
  4. It bends away from the normal because the cornea has a lower refractive index

Explanation

When light passes from a lower-index medium (air, about 1.00) to a higher-index medium (cornea, about 1.376), it refracts toward the normal according to Snell's law. Total internal reflection requires travel from a higher to a lower index at an angle beyond the critical angle, which does not apply here.

Question 14Ocular Anatomy, Physiology and Optics

Which structure forms the major site of resistance to aqueous outflow in the normal eye?

  1. The corneal epithelium
  2. The iris sphincter
  3. The ciliary processes
  4. The juxtacanalicular trabecular meshwork Correct answer.

Explanation

Most outflow resistance occurs in the juxtacanalicular region of the trabecular meshwork, adjacent to Schlemm canal. The ciliary processes produce aqueous rather than resist outflow. The iris sphincter and corneal epithelium play no role in aqueous drainage.

Question 15Ocular Anatomy, Physiology and Optics

Which visual pigment is found in rods and gives them their characteristic sensitivity?

  1. Cyanolabe
  2. Rhodopsin Correct answer.
  3. Erythrolabe
  4. Chlorolabe

Explanation

Rhodopsin, composed of opsin and the chromophore 11-cis retinal, is the rod photopigment and is most sensitive to blue-green light. Erythrolabe, chlorolabe, and cyanolabe are the cone opsins sensitive to long, medium, and short wavelengths respectively.

Question 16Ocular Anatomy, Physiology and Optics

Which cranial nerve innervates the superior oblique muscle?

  1. Oculomotor nerve (CN III)
  2. Abducens nerve (CN VI)
  3. Trochlear nerve (CN IV) Correct answer.
  4. Ophthalmic division of trigeminal nerve (CN V1)

Explanation

The trochlear nerve supplies the superior oblique. The oculomotor nerve supplies most other extraocular muscles, and the abducens supplies the lateral rectus. The ophthalmic division of the trigeminal nerve is sensory to the eye and orbit.

Question 17Ocular Anatomy, Physiology and Optics

A 60-year-old patient reports difficulty reading at near despite clear distance vision. Which age-related change best explains this?

  1. Increased corneal curvature
  2. Loss of cone photoreceptors
  3. Weakening of the medial rectus
  4. Reduced lens elasticity and capsule compliance Correct answer.

Explanation

Presbyopia results from progressive hardening of the crystalline lens and reduced capsular elasticity, limiting the ability to increase lens power for near vision. Corneal curvature changes and extraocular muscle weakness do not cause this pattern. Cone loss would affect acuity and colour rather than near focusing alone.

Question 18Ocular Anatomy, Physiology and Optics

Which of the following correctly describes the optic chiasm?

  1. All fibres from both eyes cross completely
  2. Temporal retinal fibres cross while nasal fibres remain ipsilateral
  3. Only fibres from the macula cross
  4. Nasal retinal fibres cross while temporal fibres remain ipsilateral Correct answer.

Explanation

At the chiasm, axons from the nasal hemiretinas decussate to the contralateral optic tract, while temporal fibres stay ipsilateral. This arrangement brings the left visual field to the right hemisphere and vice versa. Complete crossing or macular-only crossing does not occur.

Question 19Ocular Anatomy, Physiology and Optics

What is the approximate power of the crystalline lens in the unaccommodated adult eye?

  1. About 60 dioptres
  2. About 43 dioptres
  3. About 5 dioptres
  4. About 19 dioptres Correct answer.

Explanation

The unaccommodated crystalline lens contributes roughly 19 D, while the cornea contributes about 43 D, giving a total ocular power near 60 D. A 5 D value is far too low for the lens, and 43 D describes the cornea.

Question 20Ocular Anatomy, Physiology and Optics

A patient views an object at 25 cm. What is the approximate accommodative demand, ignoring vertex distance?

  1. 2.50 D
  2. 25.00 D
  3. 4.00 D Correct answer.
  4. 0.25 D

Explanation

Accommodative demand equals the reciprocal of the fixation distance in metres: 1/0.25 m equals 4.00 D. The demand increases as the object is brought closer. Values such as 0.25 D or 25 D misapply the conversion.

Question 21Ocular Anatomy, Physiology and Optics

Which statement about the Stiles-Crawford effect is correct?

  1. Light entering near the pupil centre is perceived brighter than light entering at the margin Correct answer.
  2. Light entering at the pupil margin is perceived brighter than central light
  3. The effect is greatest for scotopic rod vision
  4. The effect describes pupillary constriction to light

Explanation

The Stiles-Crawford effect is the directional sensitivity of cones: rays passing near the pupil centre are more effective at eliciting a brightness sensation than marginal rays. It is primarily a cone (photopic) phenomenon. It is unrelated to the pupillary light reflex.

Question 22Ocular Anatomy, Physiology and Optics

In a patient with a lesion of the left optic tract, which visual field defect is expected?

  1. Binasal hemianopia
  2. Left homonymous hemianopia
  3. Bitemporal hemianopia
  4. Right homonymous hemianopia Correct answer.

Explanation

The left optic tract carries fibres from the left temporal and right nasal hemiretinas, representing the right visual field. A lesion therefore produces a right homonymous hemianopia. Bitemporal defects arise at the chiasm, and binasal defects are uncommon and not localised to a single tract.

Question 23Ocular Anatomy, Physiology and Optics

Which factor most directly determines the depth of field of the eye?

  1. Retinal thickness
  2. Axial length
  3. Corneal thickness
  4. Pupil diameter Correct answer.

Explanation

Depth of field varies inversely with pupil diameter: a smaller pupil increases depth of field by restricting the cone of rays admitted. Axial length affects refractive error rather than depth of field directly. Corneal and retinal thickness do not govern the range of clear focus.

Question 24Ocular Anatomy, Physiology and Optics

Which best explains why the cornea is transparent despite being composed largely of collagen?

  1. Uniform collagen fibril spacing and diameter smaller than the wavelength of light Correct answer.
  2. Continuous desquamation of the anterior stroma
  3. Total absence of collagen in the stroma
  4. Presence of melanin that absorbs scattered light

Explanation

Corneal transparency depends on the regular lattice arrangement of collagen fibrils of uniform small diameter, which limits light scatter through destructive interference. The stroma is rich in collagen, so absence of collagen is incorrect. Melanin would reduce transmission, and stromal shedding does not occur.

Question 25Ocular Anatomy, Physiology and Optics

A reduced eye model assumes a single refracting surface with a power of approximately 60 D. What is its principal focal length in air?

  1. About 24 mm
  2. About 16.7 mm Correct answer.
  3. About 60 mm
  4. About 6 mm

Explanation

Focal length equals the reciprocal of power in metres: 1/60 D equals 0.0167 m, or about 16.7 mm. This is close to the axial length of the reduced eye, which is why distant objects focus near the retina. A 24 mm value would correspond to about 42 D.

Question 26Ocular Anatomy, Physiology and Optics

Which refractive component of the eye has the highest dioptric power in the unaccommodated state?

  1. The vitreous body
  2. The crystalline lens
  3. The anterior corneal surface Correct answer.
  4. The posterior corneal surface

Explanation

The air-to-cornea interface provides the largest change in refractive index in the eye, giving the anterior corneal surface roughly 48-49 D of the cornea's total ~43 D net power. The lens contributes only about 19-20 D when relaxed. The vitreous has essentially no refractive power relative to the retina it contacts.

Question 27Ocular Anatomy, Physiology and Optics

The fovea centralis achieves its high acuity mainly because of which anatomical feature?

  1. Absence of overlying retinal layers and a one-to-one cone-to-ganglion pathway Correct answer.
  2. A concentration of rods with large outer segments
  3. A high convergence ratio of photoreceptors onto ganglion cells
  4. A thick nerve fibre layer and prominent internal limiting membrane

Explanation

At the foveola the inner retinal layers are displaced laterally (the foveal pit), removing tissue that would scatter light, and each cone connects to a midget bipolar and midget ganglion cell in a private line. This low convergence preserves spatial resolution. High convergence and thick inner layers are features of peripheral retina.

Question 28Ocular Anatomy, Physiology and Optics

Aqueous humour drains from the anterior chamber into the episcleral venous system mainly via which route?

  1. Trabecular meshwork, Schlemm canal, collector channels Correct answer.
  2. Iris crypts into the posterior chamber
  3. Retinal pigment epithelium into the choroid
  4. Ciliary body stroma directly into the suprachoroidal space

Explanation

The conventional outflow pathway passes through the trabecular meshwork into Schlemm canal and then via collector channels into episcleral veins. This route carries the majority of aqueous outflow in the normal eye. The uveoscleral route is a smaller alternative pathway.

Question 29Ocular Anatomy, Physiology and Optics

Which extraocular muscle is the primary elevator of the eye in the primary position?

  1. Inferior oblique
  2. Superior oblique
  3. Inferior rectus
  4. Superior rectus Correct answer.

Explanation

Because the superior rectus has an angle of about 23 degrees to the visual axis, its main action in the primary position is elevation, with secondary intorsion and adduction. The inferior oblique also elevates but acts most strongly in adduction.

Question 30Ocular Anatomy, Physiology and Optics

A patient's near point is measured at 40 cm. What is the amplitude of accommodation, assuming a far point at infinity?

  1. 1.50 D
  2. 2.50 D Correct answer.
  3. 4.00 D
  4. 0.40 D

Explanation

Amplitude of accommodation is the difference between the vergence required at the near point and at the far point. With a far point at infinity, the near point at 0.40 m demands 1/0.40 = 2.50 D.

Question 31Ocular Anatomy, Physiology and Optics

A thin convex lens of +5.00 D is placed 2 cm in front of a +10.00 D thin lens. What is the approximate equivalent power of the combination?

  1. +14.30 D Correct answer.
  2. +5.00 D
  3. +16.00 D
  4. +15.00 D

Explanation

For separated thin lenses the equivalent power is F1 + F2 - d(F1)(F2), with d in metres. Here 5 + 10 - 0.02 x 5 x 10 = 15 - 1 = 14.00 D, and with the small separation the combination falls near 14 D.

Question 32Ocular Anatomy, Physiology and Optics

A distant object is imaged in front of the retina. Which corrective lens type and effect applies?

  1. A prism that displaces the image onto the fovea
  2. A convex lens that increases the eye's refractive power
  3. A cylindrical lens that corrects meridional power differences
  4. A concave lens that shifts the far point to infinity Correct answer.

Explanation

An image focused anterior to the retina indicates myopia, where the far point lies at a finite distance in front of the eye. A diverging (concave) lens moves the far point out to infinity so distant objects focus on the retina.

Question 33Ocular Anatomy, Physiology and Optics

Which property of the retinal image is directly reduced by higher-order aberrations?

  1. Its chromatic dispersion
  2. Its retinal illuminance
  3. Its binocular disparity
  4. Its contrast and sharpness Correct answer.

Explanation

Higher-order aberrations blur the point spread function, lowering the contrast of fine detail in the retinal image. They do not primarily change overall light level or binocular geometry.

Question 34Ocular Anatomy, Physiology and Optics

During accommodation, which set of changes occurs in the crystalline lens?

  1. Anterior chamber depth increases and lens thins
  2. Anterior surface steepens and lens thickness increases Correct answer.
  3. Equatorial diameter increases and lens flattens
  4. Lens power decreases and zonular tension rises

Explanation

Contraction of the ciliary ring releases zonular tension, allowing the elastic capsule to mould the lens into a more spherical, thicker form with a steeper anterior surface and increased power. The anterior chamber shallows as the lens thickens.

Question 35Ocular Anatomy, Physiology and Optics

Which statement correctly describes Purkinje images of a single object?

  1. The first and brightest Purkinje image arises at the anterior corneal surface Correct answer.
  2. The fourth Purkinje image arises from the anterior corneal surface
  3. All four Purkinje images are inverted relative to the object
  4. The second Purkinje image is the brightest and arises at the posterior lens surface

Explanation

Purkinje images are reflections from the optical surfaces of the eye, and the strongest is from the anterior cornea because of the large air-to-cornea index change. The fourth image arises from the posterior lens surface.

Question 36Ocular Anatomy, Physiology and Optics

Which change in pupil diameter best explains the increase in depth of focus when illumination falls?

  1. Pupil dilation increases diffraction but reduces blur circle size
  2. Pupil constriction narrows the blur circle Correct answer.
  3. Pupil constriction increases spherical aberration
  4. Pupil dilation reduces the blur circle

Explanation

A smaller pupil limits the cone of rays reaching the retina, so defocused points form smaller blur circles and the tolerable range of defocus increases. This is why squinting and bright light improve focus tolerance.

Question 37Ocular Anatomy, Physiology and Optics

A ray passes from air (n = 1.00) into the cornea (n = 1.376) at an angle of incidence of 30 degrees. What is the approximate angle of refraction?

  1. 60 degrees
  2. 42 degrees
  3. 21 degrees Correct answer.
  4. 30 degrees

Explanation

Snell's law gives sin r = (n1/n2) sin i = (1.00/1.376)(0.500) = 0.363, so r is about 21 degrees. The ray bends toward the normal when entering the denser medium.

Question 38Ocular Anatomy, Physiology and Optics

A patient has a spherical refractive error of -4.00 D at the spectacle plane, 12 mm in front of the cornea. What is the approximate ocular refraction at the corneal plane?

  1. -3.80 D
  2. -4.00 D
  3. -4.21 D Correct answer.
  4. -4.48 D

Explanation

Correction to the corneal plane uses F_c = F_s / (1 - dF_s), with d = 0.012 m and F_s = -4.00 D. This gives -4.00 / (1 + 0.048) = -3.82 D for a minus lens, and the small vertex change moves the value modestly from the spectacle power.

Question 39Ocular Anatomy, Physiology and Optics

A reduced eye model has a single refracting surface of power 60 D and a refractive index of 1.336. What is the approximate secondary focal length in the vitreous?

  1. 16.7 mm
  2. 60.0 mm
  3. 5.6 mm
  4. 22.3 mm Correct answer.

Explanation

The secondary focal length is n'/F, where n' is the vitreous index and F the surface power. Thus 1.336/60 = 0.0223 m, or about 22.3 mm, which approximates the axial length of the model eye.

Question 40Ocular Anatomy, Physiology and Optics

Which structure forms the major barrier to diffusion of large molecules from the choroid into the outer retina?

  1. Nerve fibre layer
  2. Retinal pigment epithelium tight junctions Correct answer.
  3. Bruch membrane
  4. Internal limiting membrane

Explanation

The retinal pigment epithelium forms the outer blood-retinal barrier through zonulae occludentes between its cells. These junctions regulate transport between the choriocapillaris and the photoreceptor layer.

Question 41Refraction, Retinoscopy and Prescription of Optical Aids

A patient's far point is 50 cm in front of the eye. What is the refractive error at the far point distance, expressed as the spectacle refraction needed to correct it?

  1. -2.00 D Correct answer.
  2. -0.50 D
  3. -5.00 D
  4. +2.00 D

Explanation

Refraction in dioptres equals the reciprocal of the far point distance in metres. A far point of 0.50 m gives 1/0.50 = 2.00 D of myopia, corrected with a minus lens.

Question 42Refraction, Retinoscopy and Prescription of Optical Aids

Which type of astigmatism is present when the vertical corneal meridian is steeper than the horizontal meridian?

  1. Oblique astigmatism
  2. Against-the-rule astigmatism
  3. With-the-rule astigmatism Correct answer.
  4. Irregular astigmatism

Explanation

With-the-rule astigmatism occurs when the vertical meridian has the greater refractive power, requiring a plus cylinder at or near 90 degrees. Against-the-rule is the reverse, and oblique lies away from the principal meridians.

Question 43Refraction, Retinoscopy and Prescription of Optical Aids

During retinoscopy, a with-movement is observed in all meridians. What does this indicate about the patient's refractive state before working distance is accounted for?

  1. Myopia greater than the working distance
  2. Presbyopia
  3. Astigmatism only
  4. Hyperopia or emmetropia Correct answer.

Explanation

With-movement means the far point lies behind the retinoscope, consistent with hyperopia or emmetropia. Myopia beyond the working distance produces against-movement.

Question 44Refraction, Retinoscopy and Prescription of Optical Aids

A +2.00 D working lens is used at a 50 cm working distance during retinoscopy. The neutralising lens found is +3.00 D. What is the patient's net refraction?

  1. +5.00 D
  2. +2.00 D
  3. +1.00 D Correct answer.
  4. +3.00 D

Explanation

The working distance lens must be subtracted from the neutralising lens. Subtracting +2.00 D from +3.00 D gives a net refraction of +1.00 D.

Question 45Refraction, Retinoscopy and Prescription of Optical Aids

What is the primary purpose of cycloplegic refraction in a young child?

  1. To measure corneal curvature
  2. To assess the amplitude of accommodation
  3. To relax accommodation and reveal the full hyperopic error Correct answer.
  4. To determine the near point of convergence

Explanation

Cycloplegic agents paralyse the ciliary muscle, removing active accommodation that can mask hyperopia in children. This gives a truer measure of the total refractive error.

Question 46Refraction, Retinoscopy and Prescription of Optical Aids

A patient requires a +6.00 D spectacle lens worn 12 mm from the eye. What is the approximate equivalent contact lens power at the corneal plane?

  1. +7.20 D
  2. +6.45 D Correct answer.
  3. +5.60 D
  4. +6.00 D

Explanation

For plus lenses, the effective power increases as the lens moves closer to the eye. Using the vertex distance formula, +6.00 D at 12 mm becomes about +6.45 D at the cornea.

Question 47Refraction, Retinoscopy and Prescription of Optical Aids

Which lens form is generally recommended to minimise spectacle magnification and aberrations in a high minus prescription?

  1. Strongly meniscus design with steep front curve
  2. High base curve spherical design
  3. Plano-convex design
  4. Aspheric lenticular design Correct answer.

Explanation

Aspheric lenticular designs flatten the front curve and reduce edge thickness and magnification effects in high minus lenses. This improves cosmesis and optical performance.

Question 48Refraction, Retinoscopy and Prescription of Optical Aids

What is the far point of an uncorrected myopic eye with a refractive error of -4.00 D?

  1. 4 cm in front of the eye
  2. 25 cm in front of the eye Correct answer.
  3. 40 cm in front of the eye
  4. 25 cm behind the eye

Explanation

The far point distance is the reciprocal of the refractive error in metres. For -4.00 D, 1/4 = 0.25 m, located 25 cm in front of the eye.

Question 49Refraction, Retinoscopy and Prescription of Optical Aids

A patient has a spherical equivalent of -3.00 D with -1.00 D of cylinder. What is the spherical component of the prescription?

  1. -4.00 D
  2. -2.50 D
  3. -3.00 D
  4. -3.50 D Correct answer.

Explanation

The spherical equivalent equals the sphere plus half the cylinder. Adding half of -1.00 D (-0.50 D) to -3.00 D gives -3.50 D.

Question 50Refraction, Retinoscopy and Prescription of Optical Aids

Which instrument principle underlies the duochrome (red-green) test during subjective refraction?

  1. Longitudinal chromatic aberration of the eye Correct answer.
  2. Diffraction at the pupil margin
  3. Spherical aberration of the cornea
  4. Transverse chromatic aberration of the lens

Explanation

The eye focuses green light slightly in front of red light due to longitudinal chromatic aberration. Comparing clarity on red and green targets helps refine the spherical endpoint.