Refraction and eye examination

Allied Health (Radiology / OT / Optometry / Dialysis) (B.Sc) · Optometry · free study notes for exams, placements and interviews

In short

Refraction is the clinical process of measuring the eye's refractive error and finding the lens power that gives the clearest, most comfortable vision; eye examination is the wider sequence of history, visual acuity, external and internal examination, binocular vision and intraocular pressure assessment within which refraction sits. It is the single core skill of the optometrist — every spectacle and contact lens prescription in India begins here. B.Sc Optometry syllabi (under UGC CBCS, state health universities and NCAHP-approved institutions) examine it in Optometric Optics, Clinical Refraction and Ocular Examination papers with both theory and practical/viva components. Employers test it in practical rounds: Lenskart, Titan Eyeplus, Essilor, Bausch & Lomb, Dr Agarwal's, Sankara Nethralaya, LVPEI, Aravind, Narayana Nethralaya and hospital optometry departments. Optometry in India is regulated under the National Commission for Allied and Healthcare Professions Act, 2021 (NCAHP Act), which places optometry among the recognised allied health professions with registration through State Allied and Healthcare Councils.

Explained

Refractive error: definitions and classification

Emmetropia: with accommodation relaxed, parallel rays from infinity focus exactly on the retina; far point is at infinity.
Myopia: rays focus in front of the retina; corrected by a minus (concave) lens. Axial myopia (long eye) is the common form; others are curvatural, index (nuclear cataract causing a myopic shift) and positional.
- Clinical types: simple/school myopia, degenerative (pathological, usually > −6.00 D with fundus changes), nocturnal, pseudomyopia (accommodative spasm), induced.
- Degrees: low up to −3.00 D, moderate −3.00 to −6.00 D, high above −6.00 D.
Hypermetropia (hyperopia): rays focus behind the retina; corrected by a plus (convex) lens. Components: total = manifest + latent; manifest = facultative + absolute.
Astigmatism: different power in different meridians, so a point does not image as a point but as two focal lines (Sturm's conoid) with the circle of least confusion between them.
- Regular: with-the-rule (minus cylinder axis near 180, steep vertical meridian), against-the-rule (minus cylinder axis near 90), oblique.
- Classification by focal line position: simple myopic, simple hypermetropic, compound myopic, compound hypermetropic, mixed.
- Irregular astigmatism: keratoconus, scars, pterygium — needs rigid contact lenses or topography, not spectacles.
Anisometropia: unequal refraction between eyes; aniseikonia is unequal retinal image size; antimetropia is myopia in one eye and hypermetropia in the other.
Presbyopia: age-related loss of accommodative amplitude (from about 40 years) due to lens sclerosis and ciliary changes — not a refractive error of the eye's axial optics.

The eye examination sequence

History: chief complaint (blur for distance/near, headache, asthenopia, diplopia, watering), duration, onset, present and past spectacles with age of prescription, ocular history, systemic history (diabetes, hypertension, thyroid, pregnancy), drugs, occupation, working distance, screen hours, family history of myopia/glaucoma, allergies.
Visual acuity: unaided and aided, distance and near, each eye separately then binocular; pinhole acuity if reduced.
External examination: lids, lashes, lacrimal apparatus, conjunctiva, cornea, anterior chamber by torch and slit lamp; pupil reactions (direct, consensual, RAPD by swinging-torch test).
Ocular motility and binocular vision: cover test (cover-uncover for tropia, alternate cover for phoria), extraocular movements in nine positions, near point of convergence, stereopsis.
Objective refraction: retinoscopy and/or autorefraction, keratometry.
Subjective refraction: monocular best-vision sphere, cylinder axis and power refinement, binocular balance, near addition.
Internal examination: direct and indirect ophthalmoscopy, +90 D/78 D slit-lamp fundoscopy — disc, macula, vessels, periphery.
Tonometry: Goldmann applanation (gold standard), non-contact (air-puff), Perkins, Schiotz (largely obsolete). Normal IOP 10–21 mmHg; interpret with corneal thickness.
Additional tests as indicated: colour vision (Ishihara), field screening (confrontation, perimetry), tear film (TBUT, Schirmer), amplitude of accommodation, AC/A ratio, dilated fundus examination.
Prescription, advice and recall: prescription written with sphere, cylinder, axis, add, PD, lens/coating advice, and a review date.

Visual acuity measurement

Snellen notation: VA = d/D, where d = testing distance, D = distance at which the letter subtends 5 minutes of arc. 6/6 is the standard; each letter stroke subtends 1 minute of arc at the designated distance.
Standard distance 6 m; in small Indian consulting rooms a 3 m chart with a mirror or a reversed chart is used.
Decimal VA = d/D as a fraction (6/12 = 0.5). LogMAR = log₁₀(MAR); 6/6 = 0.00, 6/60 = 1.00. ETDRS/LogMAR charts have equal letters per line, geometric size progression (0.1 log unit per line) and letter-by-letter scoring (0.02 per letter) — preferred for research and low vision.
Below chart acuity: counting fingers (CF), hand movements (HM), perception of light with projection (PL+PR), no PL.
Pinhole test: a 1–1.5 mm pinhole improves refractive and media-based blur, not retinal or neural loss.
Near vision: N-notation (N5, N6, N8) or reduced Snellen at 40 cm; Indian charts also carry regional-language reading types.
Preschool/illiterate: Landolt C, tumbling E, HOTV, Lea symbols, Cardiff cards, Teller acuity cards for infants.

Objective refraction — retinoscopy

Principle: light is streaked onto the retina and the examiner neutralises the movement of the reflex seen through the peephole, which is at the working distance.
With movement = far point behind the examiner's peephole (hypermetropia, or myopia less than the working-distance dioptre) → add plus. Against movement = far point between eye and peephole (myopia greater than working-distance dioptre) → add minus. Neutrality = far point at the peephole.
Working distance allowance: 2/3 m → subtract 1.50 D; 1/2 m → subtract 2.00 D; 1 m → subtract 1.00 D.
Types: streak retinoscopy (plane mirror effect in the divergent setting of a Welch-Allyn/Heine scope; the common clinical method), spot retinoscopy, static (accommodation relaxed, fogged or cyclopleged) and dynamic (near target, e.g. MEM and Nott methods, to assess accommodative lag).
Astigmatism signs: break, thickness, intensity, skew and width of the reflex; neutralise each principal meridian separately.
Cycloplegic refraction: essential in children, hypermetropes, accommodative esotropia, pseudomyopia, inconsistent subjective responses. Agents — atropine 1% (strongest, used in young hypermetropes/squint, cycloplegia 10–14 days), homatropine 2%, cyclopentolate 1% (clinical workhorse, refract 30–45 min, recovers in about 24 h), tropicamide 0.5–1% (weak cycloplegia, mainly mydriasis). Avoid dilation if the anterior chamber is shallow (angle-closure risk); counsel about photophobia and near blur. In India these are prescription drugs and must be used within the practitioner's scope and institutional protocol.
Post-mydriatic test (PMT): re-check subjectively after cycloplegia wears off before finalising a plus prescription.

Objective refraction — instruments

Autorefractometer: infrared optometer measuring sphere, cylinder, axis automatically; fast and reproducible, but tends to over-minus in young patients because of instrument myopia/proximal accommodation. Treated as a starting point, never a final prescription.
Keratometer (ophthalmometer): measures the central anterior corneal curvature in two meridians using doubling of mires (Javal-Schiotz, Bausch & Lomb) — gives K readings in mm and dioptres, used for corneal astigmatism, contact lens fitting and IOL power.
Corneal topography/tomography: Placido-disc or Scheimpflug mapping for keratoconus screening and irregular astigmatism.
Lensometer (focimeter/vertometer): reads the power, axis, add and optical centre of existing spectacles; neutralisation by hand lenses is the manual alternative.
Slit lamp biomicroscope: illumination types — diffuse, direct focal (optic section, parallelepiped), indirect, retro-illumination, sclerotic scatter, specular reflection; with fluorescein for staining and tear film.
A-scan biometry, OCT, fundus camera, perimeter: supporting instruments the optometrist operates in Indian hospital practice.

Subjective refraction — step by step

Occlude one eye; start from the retinoscopy/autorefraction value in a trial frame or phoropter, with correct back vertex distance (about 12–14 mm), pantoscopic tilt and centration.
Best vision sphere (BVS): add plus until acuity just begins to drop, then reduce — "maximum plus for best acuity"; use the +1.00 D blur test (should drop two to three lines) to confirm no over-minus.
Fogging: add +1.00 to +2.00 D to relax accommodation, then unfog in 0.25 D steps.
Astigmatism: find axis and power using the astigmatic fan and block, stenopaeic slit, or the Jackson cross cylinder (JCC) with the acuity reduced to about 6/12.
- JCC axis refinement: straddle the cylinder axis with the JCC handle on the axis; follow the preferred flip and rotate the cylinder axis toward the matching minus axis of the JCC, 5–10° at a time.
- JCC power refinement: handle at 45° to the axis; if the minus axis of the JCC aligned with the correcting cylinder is preferred, increase minus cylinder; compensate the sphere by +0.25 D sphere for every −0.50 D cylinder added to keep the circle of least confusion on the retina.
Duochrome (bichrome) test: red-green filters exploit chromatic aberration. Letters clearer on red = under-corrected myopia/over-plus → add minus; clearer on green = over-minus → add plus. Endpoint is equality. Invalid in acuity worse than about 6/9 but valid even in colour-defective patients (the test depends on chromatic aberration, not on colour discrimination; only very poor acuity invalidates it).
Binocular balance: prism dissociation (3Δ base-up/base-down) or alternate occlusion with +1.00 D fog, equalising blur between eyes; then binocular refinement of the sphere.
Near addition: estimate from age or from half the remaining amplitude of accommodation, cross-check with NRA/PRA (near point balance) and the patient's own working distance; prescribe the weakest add that gives comfortable clearance.
Trial frame tolerance: let the patient walk and read with the final correction; check cylinder changes, anisometropia and prism tolerance.

Writing and checking the prescription — Indian practice

Format: Sphere / Cylinder × Axis, Add, with PD and distance; axis in the TABO convention 1–180°, measured anticlockwise as seen by the examiner facing the patient, starting from the horizontal on the examiner's right (the patient's left — nasal for the right eye, temporal for the left). OD = right, OS = left, OU = both.
Minus-cylinder form is the clinical standard in India; plus-cylinder form is used by some ophthalmology departments and lab systems — transposition converts between them.
PD (interpupillary distance): measured by PD rule or pupillometer; distance PD typically 58–66 mm in adults, near PD about 3–4 mm less; monocular PDs needed for progressives.
Fitting measurements: optical centre height, fitting/segment height, frame wrap and tilt — a decentration error induces unwanted prism (Prentice's rule).
Vertex distance compensation matters above about ±4.00 D: a minus lens moved closer to the eye needs more minus power; a plus lens moved closer needs more plus.
Standards and compliance: ophthalmic lenses, frames and uncut finished lenses are covered by Indian Standards (IS specifications for spectacle lenses and frames, aligned with ISO 8980 and ISO 12870 series); spectacle lenses and frames fall under BIS/Quality Control Order requirements that have been expanded in recent years — verify the current QCO and IS edition before quoting, as these notifications change. Spectacles and goggles attract GST at 5% following the September 2025 rate revision (GST slabs are now 5%, 18% and 40%); confirm the current HSN-wise rate before billing.
Records: prescriptions and clinical records are kept as per hospital policy and the Act-based professional conduct expected of a registered allied health professional; patient data handling comes under the Digital Personal Data Protection Act, 2023.
Optometrists in India perform refraction, dispense and manage refractive and binocular vision care; diagnosis and treatment of ocular disease and prescribing of therapeutic drugs remain with ophthalmologists — scope is defined by the NCAHP Act framework and state council rules.

Key points

Refraction - determining the refractive error and the lens power that gives best corrected acuity comfortably.
Emmetropia - parallel rays focus on the retina with accommodation relaxed; far point at infinity.
Myopia / hypermetropia - image before / behind the retina; minus / plus correction respectively.
Sturm's conoid - the interval between the two focal lines of an astigmatic eye; circle of least confusion lies at the dioptric mean.
Maximum plus, minimum minus - the governing rule of subjective refraction; prevents accommodative over-minus.
Snellen VA = d/D - 6/6 means the letter subtending 5' of arc at 6 m is read at 6 m.
LogMAR - log of the minimum angle of resolution; 6/6 = 0.00, each line = 0.1 log unit, lower is better.
Retinoscopy - objective neutralisation of the reflex; "with" movement → add plus, "against" → add minus.
Working distance correction - subtract 1.50 D at 2/3 m, 2.00 D at 1/2 m from the gross retinoscopy finding.
Cycloplegic refraction - mandatory in children, hypermetropes and squint; cyclopentolate 1% is the usual agent, atropine the strongest.
Autorefractor - fast objective starting point; over-minuses young patients, never the final prescription.
JCC - refines cylinder axis then power; add +0.25 D sphere per −0.50 D cylinder increase.
Duochrome - red clearer → add minus; green clearer → add plus; needs acuity better than about 6/9.
Binocular balance - prism dissociation or alternating fog equalises accommodative demand between eyes.
Spherical equivalent = sphere + cylinder/2 - used to compare or convert prescriptions.
Transposition - add cylinder to sphere, reverse the cylinder sign, shift axis by 90°.
Prentice's rule - prism (Δ) = decentration (cm) × lens power (D); explains dispensing discomfort.
Near add - weakest plus giving comfortable clarity at the patient's own working distance; presbyopia begins about 40 years.
Normal IOP - 10–21 mmHg; Goldmann applanation is the reference method.
NCAHP Act, 2021 - statutory framework for optometry registration and scope of practice in India.

Formulas and examples

Refractive error of the eye: F = 1/f, where F is power in dioptres (D) and f the focal length in metres. Lens power is positive for converging, negative for diverging lenses.
Far point distance (myopia): correcting lens power F = −1/k, where k is the far point distance in metres from the spectacle plane.
Snellen acuity = d/D (d = test distance, D = distance at which the letter subtends 5′). LogMAR = log₁₀(MAR), MAR in minutes of arc = D/d for a 6-metre reference.
Net retinoscopy = gross finding − working-distance dioptre (1/working distance in metres).
Spherical equivalent SE = Sph + (Cyl / 2), in dioptres.
Transposition: new Sph = Sph + Cyl; new Cyl = −Cyl; new Axis = Axis ± 90 (keep within 1–180°).
Amplitude of accommodation A = 1/near point (m) − 1/far point (m), in dioptres; Hofstetter minimum A = 15 − 0.25 × age.
Near add = accommodative demand at working distance − (half the amplitude of accommodation), in dioptres.
Effective power at a new vertex distance: F₂ = F₁ / (1 − d·F₁), where d is the shift toward the eye in metres (positive when the lens moves closer).
Prentice's rule: prism P (prism dioptres, Δ) = c × F, c = decentration in cm, F = lens power in D.
Accommodative demand at distance x (m) = 1/x dioptres (40 cm → 2.50 D).

Example 1 — Net retinoscopy with astigmatism. A student at 2/3 m working distance neutralises the right eye at +1.50 D in the 180° meridian and +0.50 D in the 90° meridian.

Working-distance dioptre = 1/0.667 = 1.50 D; subtract from both meridians.
Net: 0.00 D along 180°, −1.00 D along 90°.
Power along 90° is more minus, so the correcting minus cylinder axis lies along 180°.
Prescription (minus-cyl form): 0.00/−1.00 × 180 — simple myopic astigmatism, with-the-rule.
Spherical equivalent = 0.00 + (−1.00/2) = −0.50 D.

Example 2 — Transposition and spherical equivalent. Given +2.50/−1.50 × 70.

New sphere = +2.50 + (−1.50) = +1.00.
New cylinder = +1.50.
New axis = 70 + 90 = 160.
Plus-cyl form: +1.00/+1.50 × 160. Spherical equivalent = +2.50 − 0.75 = +1.75 D (same from the plus form: +1.00 + 0.75 = +1.75 D).

Example 3 — Near addition for a presbyope. A 48-year-old emmetrope reads at 33 cm; measured amplitude of accommodation is 4.00 D.

Accommodative demand = 1/0.33 = 3.00 D.
Reserve half the amplitude: usable = 4.00/2 = 2.00 D.
Add required = 3.00 − 2.00 = +1.00 D; range of clear vision checked with +1.00 D before issuing.
Hofstetter minimum amplitude = 15 − 0.25 × 48 = 3.00 D, so this patient's 4.00 D is above the expected minimum.

Example 4 — Vertex distance compensation. A trial-frame finding of −10.00 D at 14 mm vertex distance is to be fitted in a contact lens (vertex 0 mm, i.e. moved 14 mm closer).

F₂ = F₁/(1 − dF₁) = −10.00 / (1 − 0.014 × (−10.00)) = −10.00 / 1.14 = −8.77 D → order −8.75 D.
For a +10.00 D finding: F₂ = +10.00 / (1 − 0.014 × 10.00) = 10.00/0.86 = +11.63 D → +11.50 D (plus lenses need more plus closer to the eye).

Example 5 — Prentice's rule in dispensing. A −6.00 D lens is mounted with the optical centre 4 mm nasal to the pupil centre.

c = 0.4 cm, F = 6.00 D → P = 0.4 × 6.00 = 2.4Δ of unwanted prism per eye.
Both eyes decentred the same way in-wards for a minus lens give base-out prism, so up to about 4.8Δ total — a common cause of "the new glasses pull my eyes" complaints; remedy is correct monocular PD and optical centre height.

Test	What it measures	Objective/Subjective	Key limitation
Retinoscopy	Refractive error of each meridian	Objective	Needs skill; accommodation must be controlled
Autorefraction	Sphere, cylinder, axis	Objective	Over-minuses young patients; poor with media opacity
Keratometry	Anterior corneal curvature, corneal astigmatism	Objective	Central 3 mm only; misses irregular periphery
BVS + JCC	Final sphere and cylinder	Subjective	Needs patient cooperation and acuity ≥ 6/12
Duochrome	Residual sphere error	Subjective	Invalid below about 6/9; affected by pupil size
Pinhole	Whether blur is refractive/media or retinal	Objective aid	Reduces illumination; diffraction at < 1 mm

Astigmatism type	Focal lines relative to retina	Example (minus-cyl)
Simple myopic	One on retina, one in front	0.00/−1.50 × 180
Simple hypermetropic	One on retina, one behind	+1.50/−1.50 × 180
Compound myopic	Both in front	−2.00/−1.00 × 180
Compound hypermetropic	Both behind	+2.00/−1.00 × 180
Mixed	One in front, one behind	+1.00/−2.50 × 180

Where it is used

Retail optical chains: Lenskart, Titan Eyeplus, Specsmakers, Vision Express — refraction, dispensing, lens and coating advice, store-level autorefractor and lensometer work.
Hospital and eye-care institutes: LVPEI, Aravind, Sankara Nethralaya, Dr Agarwal's, AIIMS and medical-college eye OPDs — pre-consultation workup, refraction clinics, paediatric and low-vision clinics, cataract biometry, OCT and perimetry.
Community and school vision screening: NPCB&VI (National Programme for Control of Blindness and Visual Impairment) refractive error and school eye health programmes, vision centres, teleoptometry hubs and mobile units.
Industry and corporate roles: Essilor, Zeiss, Hoya, Bausch & Lomb, Alcon, CooperVision — lens design support, technical training, clinical affairs and product demonstration; occupational vision screening for drivers, pilots and machine operators.
Refractive surgery and IOL workup: keratometry, topography, biometry and cycloplegic refraction for LASIK/SMILE and cataract surgery planning.
Academia and research: B.Optom/M.Optom teaching, clinical trials, myopia-control studies — a fast-growing area given India's rising school myopia prevalence.

Interview questions

Q: Why do you subtract 1.50 D after retinoscopy at 2/3 m? A: At neutrality the patient's far point is at the peephole, 2/3 m away, which is 1.50 D of vergence; subtracting it converts the finding to the distance (infinity) correction.
Q: A patient reads 6/6 with your finding but the autorefractor gave 0.50 D more minus. Which do you prescribe? A: The lower minus — maximum plus for best acuity; the extra minus is usually instrument-induced accommodation and causes asthenopia.
Q: When is cycloplegic refraction compulsory? A: In children and young hypermetropes, suspected pseudomyopia or accommodative spasm, accommodative esotropia, and whenever subjective responses are variable or do not match the objective finding.
Q: Transpose −3.00/+1.00 × 90. A: Sphere −3.00 + 1.00 = −2.00; cylinder −1.00; axis 90 − 90 = 180, so −2.00/−1.00 × 180.
Q: What does the duochrome test tell you if the red side is clearer? A: The eye is under-minused or over-plussed — the focus sits in front of the retina, and the longer-wavelength red image falls nearer the retina than the green — so add minus in 0.25 D steps until red and green are equal.
Q: How do you decide the near add in practice? A: Start from age or half the remaining amplitude of accommodation, verify with NRA/PRA balance, then confirm at the patient's own habitual working distance and prescribe the weakest comfortable plus.
Q: Why does a high myope need less minus power in a contact lens than in spectacles? A: Moving a minus lens closer to the eye increases its effective minus power, so a weaker lens suffices; F₂ = F₁/(1 − dF₁).
Q: Patient complains the new glasses "pull" after a cylinder axis change. What do you check? A: Confirm the axis and power against the old pair on a lensometer, check monocular PD, optical centre height and tilt for induced prism, and explain the adaptation period; recheck if symptoms persist beyond a week or two.
Q: VA is 6/24 and improves to 6/9 with a pinhole. What does that suggest? A: The loss is refractive or due to media irregularity rather than retinal or optic nerve disease — refract carefully and assess the cornea and lens.
Q: What is the difference between with-the-rule and against-the-rule astigmatism? A: With-the-rule has the steeper (more powerful) meridian vertical, giving a minus cylinder axis near 180; against-the-rule has the steeper meridian horizontal, with the minus cylinder axis near 90.
Q: Why is LogMAR preferred over Snellen in research? A: Equal letter counts per line, uniform 0.1 log-unit steps and letter-by-letter scoring make it linear, more sensitive and statistically usable.
Q: A 55-year-old myope says she reads better without glasses. Why? A: Uncorrected myopia brings the far point to a near distance, so she needs little or no accommodation to read — her myopia substitutes for the presbyopic add.
Q: What are the limits of the optometrist's role here under Indian law? A: Under the NCAHP Act, 2021 framework an optometrist refracts, dispenses and manages refractive and binocular vision care and performs diagnostic workups, while diagnosing and treating ocular disease and prescribing therapeutic drugs remain with the ophthalmologist; exact scope follows the central and state council rules.

Common mistakes

Forgetting the working-distance subtraction, or applying it to only one meridian in astigmatic retinoscopy.
Over-minusing — accepting the strongest minus the patient "likes" instead of the least minus giving best acuity; skipping the +1.00 D blur check.
Transposition slips: changing the cylinder sign but not adding it to the sphere, or taking the axis outside the 1–180° range.
Refracting a child without cycloplegia and reporting latent hypermetropia as emmetropia or false myopia.
Refining the cylinder with JCC without compensating the sphere (+0.25 D sphere per −0.50 D cylinder), so the circle of least confusion drifts off the retina.
Ignoring vertex distance in powers above about ±4.00 D, or writing a spectacle power unchanged onto a contact lens order.
Treating the autorefractor printout or an unchecked old prescription as the final answer, and skipping monocular PD, centration height and a binocular balance.
Finishing at refraction alone — no cover test, pupils, slit lamp, fundus or IOP — and missing cataract, glaucoma, amblyopia or diabetic retinopathy behind the blur.