What Is Emmetropia?
Emmetropia is the ideal refractive state where the eye focuses distant light precisely on the retina without any accommodative effort. An emmetropic eye has a perfect match between its optical power (cornea + crystalline lens, approximately 60 D total) and its axial length (approximately 24 mm).
Emmetropia is the opposite of ametropia (any refractive error: myopia, hyperopia, or astigmatism). In practice, very few eyes are perfectly emmetropic; most have small amounts of refractive error that may or may not require correction.
Visual Acuity: Measuring How Well You See
Visual acuity (VA) is a quantitative measure of the eye's ability to resolve fine spatial detail. It is the most commonly used clinical measurement of visual function.
The Snellen Chart
The standard Snellen chart uses letters (optotypes) of progressively smaller sizes. Visual acuity is expressed as a fraction:
VA = Testing distance / Distance at which a normal eye can read that line
For example, 20/40 means the patient must be at 20 feet to read what a normal eye can read at 40 feet. The letters are twice the size needed for normal acuity.
Common VA Values
| Acuity | Meaning | Clinical Significance |
|---|---|---|
| 20/20 (6/6) | Normal acuity | Standard for driving and daily tasks |
| 20/15 | Better than normal | Can resolve finer detail than average |
| 20/40 | Mildly reduced | Minimum for unrestricted driving in many states |
| 20/70 | Moderately reduced | May need large print |
| 20/200 | Legal blindness (with best correction) | Largest Snellen letter at 20 feet |
What Determines Visual Acuity?
Visual acuity depends on several factors:
- Optical focus: Refractive errors must be corrected for best acuity
- Retinal health: Macular disease directly reduces central acuity
- Neural processing: The visual pathway from retina to cortex must function properly
- Pupil size: Affects depth of focus and aberrations
- Illumination: Better lighting improves contrast and acuity
- Contrast: Standard charts use high-contrast black on white
Minimum Angle of Resolution (MAR)
The MAR is the angular size of the smallest detail the eye can resolve. For 20/20 acuity, the MAR is 1 minute of arc (1/60 of a degree). Each Snellen letter subtends 5 minutes of arc at its designated distance, with stroke width of 1 minute of arc.
LogMAR (logarithm of the MAR) is an alternative notation used in research and some clinical settings:
- 20/20 = LogMAR 0.0
- 20/40 = LogMAR 0.3
- 20/200 = LogMAR 1.0
Recording Visual Acuity
Visual acuity is recorded for each eye and both eyes together:
- sc (sine correctione) = without correction
- cc (cum correctione) = with correction
- PH = pinhole (if acuity improves with pinhole, the cause is likely refractive)
Example notation: VA OD 20/40 sc, 20/20 cc means the right eye sees 20/40 without glasses and 20/20 with glasses.
Key Takeaways
- Emmetropia is the refractive state where light focuses on the retina without accommodation
- Visual acuity is recorded as a Snellen fraction (testing distance / readable line distance)
- 20/20 represents 1 minute of arc resolution, the normal standard
- VA depends on optics, retinal health, neural processing, and testing conditions
- 20/20 measures only central, high-contrast resolution, not overall visual health