Inside the eye
A horizontal section through the right eye, seen from above (front of the eye on the left, nose towards the bottom). Click structures, zoom into the front of the eye, the back of the eye, and down to the cell layers of the retina.
Focus, refractive errors & spectacles
Real refraction (Snell's law) through a four-surface schematic eye (Le Grand). Move the object, age the eye, lengthen or shorten it, add astigmatism, then prescribe a spectacle lens that puts the image back on the retina.
What this eye sees
Live results
D (dioptre) = 1 / focal length in metres, so a stronger lens has more dioptres. DS = dioptres of sphere, the lens you would prescribe (minus for myopia, plus for hyperopia). Blur is in arcmin (1/60 of a degree); 6/6 (20/20) vision resolves detail of about 1 arcmin.
Rods, cones and the blind spot
Photoreceptors are not spread evenly. Drag along the retina to see how cone and rod density change with eccentricity, then find your own blind spot.
Three cones and colour blindness
Pick a wavelength to see how strongly each cone type responds, then view a scene and a dot plate as people with different colour vision deficiencies would see them. Drag the divider to compare.
Key facts to remember
- Visual acuity is written test distance / distance at which a normal eye reads that line: 6/18 means the patient reads at 6 m what a normal eye reads at 18 m. Test each eye separately, right first, with distance glasses on.
- Pinhole: if acuity is worse than 6/12 and improves with a pinhole, the cause is refractive error; if not, look for cataract, corneal scarring or retinal disease.
- WHO categories (presenting acuity, better eye): mild impairment worse than 6/12; moderate worse than 6/18; severe worse than 6/60; blindness worse than 3/60.
- Conversions: decimal acuity = 6 ÷ denominator; logMAR = log₁₀(denominator ÷ 6). 6/6 → decimal 1.0, logMAR 0; 6/12 → 0.5, logMAR 0.3; 6/60 → 0.1, logMAR 1.0.
- Lenses: power (D) = 1 / focal length (m). Myopia: minus (concave) lens; hyperopia and presbyopia: plus (convex). A contact lens for a far point at x m is −1/x D.
- Cataract (cloudy lens) is the leading cause of blindness in Kenya: gradual, painless loss, absent or dull red reflex. Treated by surgery with an intraocular lens.
- Trachoma (Chlamydia trachomatis, spread by eye-seeking flies, hands and cloths) is endemic in dry areas where water is scarce; in Kenya it is concentrated in the arid pastoral counties. SAFE: Surgery for trichiasis, Antibiotics (azithromycin mass treatment), Facial cleanliness, Environmental improvement.
- Vitamin A deficiency: night blindness (rods need retinal) → Bitot's spots → corneal xerosis → keratomalacia. Routine supplements 100,000 IU at 6–11 months and 200,000 IU every 6 months from 12–59 months.
Sources: WHO ICD-11 vision impairment categories; WHO trachoma fact sheet and SAFE strategy; Kenya Trachoma Elimination Programme reports; WHO vitamin A supplementation guidance and MSF clinical guidelines (xerophthalmia).
The eye as an optical instrument
Light is refracted at four surfaces before reaching the retina: the front and back of the cornea and the front and back of the crystalline lens. The cornea provides about two-thirds of the eye's power (≈ 43 D), because the jump in refractive index from air (1.000) to cornea (1.376) is by far the largest. The lens (≈ 20 D relaxed) is the adjustable part. The whole relaxed Le Grand schematic eye used in Lab 2 has a power of 59.9 D and an axial length of 24.2 mm.
We describe light by its vergence $L = n/l$, where $l$ is the signed distance to the point the light comes from or goes to (negative for real objects in front of the lens). A thin lens adds its power: $L' = L + P$. Light from an object 25 cm away has $L = -4$ D; from 6 m or more, $L \approx 0$.
Accommodation
To focus near objects the ciliary muscle contracts, the zonules slacken and the elastic lens becomes rounder (mainly the front surface), increasing its power. The amplitude of accommodation falls with age as the lens stiffens; Hofstetter's average formula is $A \approx 18.5 - 0.3\times\text{age}$ D. The far point is the farthest point seen clearly with relaxed accommodation; the near point is the closest with maximum accommodation:
Here $R_x$ is the refractive error expressed as the correcting power (negative for myopes, positive for hyperopes), so a −2.50 D myope has its far point where light has vergence −2.50 D, i.e. 40 cm in front of the eye.
Refractive errors
| Condition | Cause | Image of distant object | Correction |
|---|---|---|---|
| Myopia (short-sight) | Eye too long or too powerful | In front of the retina | Diverging (concave, minus) lens |
| Hyperopia (long-sight) | Eye too short or too weak | Behind the retina (young people can accommodate to overcome it) | Converging (convex, plus) lens |
| Presbyopia | Age-related loss of accommodation | Distance fine, near blurred | Plus "reading addition", bifocals or varifocals |
| Astigmatism | Cornea (or lens) more curved in one meridian, like a rugby ball | Two focal lines instead of a point | Cylindrical (toric) lens |
A correcting lens must form an image of a distant object at the eye's far point. For a spectacle lens at vertex distance $d$ from the cornea and a far point at distance $l_\text{far}$ from the cornea:
Each 1 mm of extra axial length makes an eye about 2.7 D more myopic. Most myopia is axial; its rapid rise worldwide is linked to more near work and less time outdoors in childhood.
Astigmatism
In an astigmatic eye one meridian (say vertical) is more curved and focuses in front of the other. A point becomes two perpendicular focal lines separated by the "interval of Sturm", with a circle of least confusion between them. A cylindrical lens has power only perpendicular to its axis, so a −2.00 D cylinder at axis 180° weakens just the vertical meridian. Prescriptions are written sphere / cylinder × axis, e.g. −1.50 / −2.00 × 180. In Lab 2 the upper rays show the vertical meridian and the lower rays the horizontal meridian.
The pupil and depth of field
The iris sphincter (parasympathetic) constricts the pupil in bright light; the dilator (sympathetic) widens it in the dark, from about 2 to 8 mm (Lab 2 uses the Moon–Spencer formula $D = 4.9 - 3\tanh(0.4\log_{10}L)$ mm, $L$ in cd/m²). The angular size of a blur circle is roughly pupil diameter × defocus: $\beta \approx D_\text{pupil}\,|\Delta L|$ radians. Small pupils give a larger depth of field, which is why people with uncorrected myopia squint and why reading is easier in bright light.
The retina
The retina is "inverted": light passes through the ganglion and bipolar cell layers before reaching the photoreceptors, which face the pigment epithelium. Two photoreceptor systems give us duplex vision:
- Rods (≈ 92 million): very sensitive (can respond to a single photon), one pigment (rhodopsin, peak ≈ 498 nm), dim-light (scotopic) vision, no colour, absent from the central fovea, peak density about 20° out.
- Cones (≈ 4.6 million): daylight (photopic) vision, high acuity and colour; three types with pigment peaks near 420 nm (S), 534 nm (M) and 564 nm (L). Packed at up to ~200,000/mm² in the fovea, where the inner layers are pushed aside to form a pit.
About 1.2 million ganglion cell axons converge at the optic disc, where there are no photoreceptors: the blind spot, about 15° nasal to the fovea (so it appears 15° to the temporal side in the visual field). The brain fills it in, so you never notice it.
Clinical relevance
Measuring visual acuity. Use a Snellen chart, or a Tumbling E chart for children and people who cannot read letters, at 6 m (or a 3 m chart) in good light. Test the right eye first with the left covered by an occluder or the palm (not fingers that can peep), then the left. Record the smallest line read as a fraction, e.g. 6/18. If the top (6/60) line cannot be read, move the patient closer: reading it at 3 m is 3/60. Below that, record counting fingers (CF) at a distance, hand movements (HM), perception of light (PL) or no perception of light (NPL). Lab 2's chart blurs exactly as an uncorrected eye would.
Cataract is clouding of the crystalline lens, mostly with age (also diabetes, steroids, trauma, UV light). Light scatters instead of focusing, so acuity falls even though the eye's length is normal; a pinhole does not help much. Surgery removes the lens and replaces its ≈ 20 D with a plastic intraocular lens (IOL). Without an IOL the eye is aphakic and very hyperopic, needing about +10 D spectacles.
Trachoma is repeated chlamydial conjunctivitis. Inflammation (TF: follicles on the upper tarsal conjunctiva in children) scars the lid (TS), which turns the lashes inward (trichiasis, TT); the lashes scratch and scar the cornea (CO), causing irreversible blindness. Elimination as a public health problem requires TF below 5 % in children aged 1–9 years and TT below 0.2 % in adults aged 15 years and over (WHO).
Vitamin A deficiency (xerophthalmia) is most common in young children with measles, diarrhoea or malnutrition. Rhodopsin in rods is made from retinal (vitamin A), so the first symptom is night blindness: rods fail first (Lab 3 shows rods dominate the periphery). Treatment is high-dose vitamin A on day 1, day 2 and again at least 2 weeks later (WHO; MSF uses day 8); corneal lesions are an emergency.
Refractive error is the commonest cause of poor vision in schoolchildren and is fully treatable with spectacles. Uncorrected presbyopia means a tailor, mechanic or nurse can no longer do close work after about 45: simple reading glasses restore it.
Colour vision and colour blindness
Colour comes from comparing the outputs of the three cone types (trichromacy) and is then recoded as opponent channels: red–green (L − M), blue–yellow (S − (L + M)) and light–dark (L + M). The genes for the L and M pigments lie on the X chromosome, so red–green deficiencies are X-linked and far commoner in males (about 8 % of men of northern European ancestry and about 2–4 % in many African populations, against < 0.5 % of women). Protan defects affect L cones, deutan defects M cones and tritan defects (rare, not sex-linked) S cones.
Visual acuity converter
Enter a Snellen fraction to get decimal acuity, MAR, logMAR and the WHO category (if this is the better eye).
Lens from the far point
For a myope who sees clearly only up to a certain distance, find the correcting lens at the spectacle plane and as a contact lens.
Practice drill
Randomised questions with instant feedback. Get five right in a row to complete the challenge.
Worked examples
1. Prescribing for a myope
- A student sees clearly only up to 40 cm: far point $l_\text{far} = -0.40$ m from the cornea.
- Spectacle lens 12 mm in front: $F = 1/(l_\text{far} - d) = 1/(-0.40 + 0.012) = 1/(-0.388) = -2.58$ D, prescribed as −2.50 D (nearest 0.25 D).
- A contact lens sits on the cornea ($d = 0$): $F = 1/(-0.40) = -2.50$ D. Minus lenses need slightly less power the closer they are to the eye.
2. Axial myopia in Lab 2
- Set the axial length to 25.6 mm (1.4 mm longer than the emmetropic 24.2 mm).
- The lab reports a prescription of about −3.75 D, i.e. ≈ −2.7 D per millimetre.
- Far point at the spectacle plane: $1/3.73 = 0.268$ m, i.e. about 28 cm from the cornea. Wear −3.75 D and the image returns to the retina.
3. Presbyopia and a reading addition
- Age 50: $A \approx 18.5 - 0.3\times50 = 3.5$ D. Near point $= 1/3.5 = 0.29$ m.
- Reading at 40 cm needs 2.5 D. Comfortably we use only about half the amplitude (1.75 D), so we need an addition of $2.5 - 1.75 = +0.75$ D.
- At 60, $A \approx 0.5$ D: an addition of about +2.25 D is needed to read at 40 cm, and about +3.75 D for 25 cm: the page is only 23.8 cm from a spectacle lens worn 12 mm in front of the eye, so the lens must supply $1/0.238 - 0.5 \approx 3.7$ D (try it in Lab 2: age 60, object 25 cm, auto-accommodation on).
4. How blurred is 1 dioptre?
- Pupil 4 mm, defocus 1 D: $\beta = 0.004 \times 1 = 0.004$ rad $= 0.004\times\frac{180}{\pi}\times60 = 13.8$ arcmin.
- Normal acuity (6/6) resolves 1 arcmin detail: 1 D of blur is very noticeable. With a 2 mm pupil it halves to 6.9 arcmin.
5. Focal length of the eye
- Equivalent power 59.9 D. In air $f = 1/59.9 = 16.7$ mm.
- Inside the eye (vitreous $n' = 1.336$): $f' = n'/P = 1.336/59.9 = 22.3$ mm behind the second principal point, which lies about 1.9 mm behind the cornea, putting the focus at 24.2 mm: on the retina.
6. Recording and classifying acuity
- A patient's right eye reads down to the 6/24 line, the left eye to 6/60. Decimal: $6/24 = 0.25$ and $6/60 = 0.1$.
- $\text{logMAR} = \log_{10}(24/6) = \log_{10}4 = 0.60$ for the right eye and $\log_{10}10 = 1.00$ for the left.
- WHO categories use the better eye: 6/24 is worse than 6/18 but not worse than 6/60, so moderate vision impairment. With a pinhole the right eye improves to 6/9: the main cause is refractive error, and spectacles will help.
7. Vitamin A for xerophthalmia (teaching example)
- A 3-year-old with night blindness and Bitot's spots: WHO treatment for children 12 months and over is 200,000 IU by mouth on day 1, day 2 and again at least 2 weeks later.
- Infants 6–11 months receive 100,000 IU per dose and those under 6 months 50,000 IU.
- One 200,000 IU capsule usually holds about 8 drops, so a 100,000 IU dose is half the capsule (about 4 drops) squeezed into the mouth.
Clinical scenarios
Work through each patient one step at a time. Lab buttons set up the optics bench or the retina so you can see the problem.
OSCE station: visual acuity
Measure and record distance visual acuity with a Snellen or Tumbling E chart, as in a clinical practical exam. The mark sheet appears at the end.
Challenges check themselves as you work. Each light turns green when done.