Visual Optics for Optometrists: Schematic Eye, Purkinje Images, Pupil, Aberrations and Depth of Focus
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
Visual optics treats the eye itself as an optical instrument. The numbers of the schematic eye, how the pupil changes image quality, and why acuity has a limit come up in every B.Optom exam, and they explain everyday clinic findings such as night myopia and the pinhole effect.
The schematic and reduced eye
| Value (Gullstrand-type exact eye) | Approximate figure |
|---|---|
| Total power of the eye | About +58.6 to +60 D |
| Corneal power | About +43 D (anterior surface about +48 D, posterior about −6 D) |
| Crystalline lens power (unaccommodated) | About +19 to +21 D |
| Axial length | About 24 mm |
| Anterior chamber depth | About 3.0–3.6 mm |
| Refractive indices | Cornea 1.376, aqueous and vitreous 1.336, lens about 1.386 (cortex) to 1.406 (nucleus) |
| Nodal points | About 7 mm behind the cornea, close to the back of the lens |
The reduced eye simplifies the eye to one refracting surface of about +60 D with a single nodal point about 17 mm in front of the retina. It lets you estimate retinal image size: image size = object size × 17 mm ÷ object distance (both in mm). A 1 mm change in axial length changes refraction by about 3 D.
Purkinje images
| Image | Formed by | Appearance |
|---|---|---|
| I | Anterior corneal surface | Upright, virtual, brightest; used in keratometry and the Hirschberg test |
| II | Posterior corneal surface | Upright, faint, almost overlapping I |
| III | Anterior lens surface | Upright, larger and dimmer; moves forward and becomes smaller in accommodation |
| IV | Posterior lens surface | Inverted, small (the posterior lens acts as a concave mirror) |
The pupil and image quality
- Small pupils increase depth of focus and reduce spherical aberration, but below about 2 mm diffraction limits resolution. This is the pinhole effect: a pinhole of about 1.2 mm improves vision blurred by refractive error.
- Large pupils (dim light) increase spherical aberration, causing night myopia (about −0.50 to −1.00 D); tonic accommodation in the dark adds to it.
- Stiles-Crawford effect: light entering the centre of the pupil is more effective than light entering the edge, because cones are oriented towards the pupil centre; this reduces the effect of peripheral aberrations.
- Depth of field and depth of focus: depth of field is the range of object distances seen clearly; depth of focus is the corresponding range at the retina. Both increase with smaller pupils.
Why visual acuity has a limit
- Foveal cones are about 2–2.5 µm apart. Resolving two points needs an unstimulated cone between them, which corresponds to about 1 minute of arc: the basis of 6/6 (20/20) letters, which subtend 5 minutes of arc with 1-minute details.
- The best young eyes reach about 6/4 to 6/3. Acuity falls with age, small pupils, defocus, scatter (cataract) and reduced contrast.
- Contrast sensitivity measures vision across sizes and contrasts; it can drop in cataract, glaucoma and optic neuropathy while Snellen acuity is still good.
Refractive error from an optics point of view
| Type | Optics | Where the far point lies |
|---|---|---|
| Myopia | Too much power for the length (axial or refractive) | In front of the eye, at a finite distance (−2.00 D: 50 cm) |
| Hyperopia | Too little power; latent part hidden by accommodation | Behind the eye (virtual) |
| Astigmatism | Different power in two meridians; regular (principal meridians at 90°) or irregular | Two far points |
| Anisometropia | Different refraction in the two eyes; risk of aniseikonia and amblyopia | Different in each eye |
| Presbyopia | Loss of accommodation with age | Near point recedes beyond the reading distance |
Knapp's law: for axial ametropia, a spectacle lens placed at the anterior focal point of the eye (about 15 mm in front of the cornea) gives a retinal image the same size as in an emmetropic eye. This is why spectacles suit axial anisometropia and contact lenses suit refractive anisometropia, although in practice contact lenses are often better tolerated.
Practical pearls
- A patient who sees well in daylight but poorly when driving at night may have night myopia: try −0.50 D extra for night driving after excluding cataract and uncorrected astigmatism.
- If vision improves with a pinhole, the problem is mainly optical (refraction or media); if it does not, think macula or optic nerve.
- Purkinje image III moving forward is the objective sign that accommodation is happening.
Where students slip
- Saying the lens has more power than the cornea: the cornea provides about two-thirds of the eye's power.
- Forgetting that diffraction worsens vision when the pupil is very small.
- Applying Knapp's law rigidly: most anisometropes do better in contact lenses whatever the type.
Test yourself: flip cards
Quick quiz
Related: Clinical Optics Made Easy: Vergence, Prisms and Magnification, Ocular Physiology In Depth: Tear Film, Cornea, Aqueous, Lens, Retina and Vision, 6/60, 6/36, 6/18 Vision Meaning and Glasses Power, Understanding Myopia, Visual Acuity Conversion Chart: Snellen, logMAR, Decimal and Near. Back to the optometry curriculum.
Key sources
- Atchison DA, Smith G. Optics of the Human Eye. Butterworth-Heinemann; 2000
- Rabbetts RB. Bennett and Rabbetts' Clinical Visual Optics. 4th ed. Butterworth-Heinemann; 2007
- American Academy of Ophthalmology. BCSC Section 3: Clinical Optics and Vision Rehabilitation (current edition)
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