Refractometry

Refractometry is 5% of the COA exam: retinoscopy, subjective refinement, and the technique of finding, for each eye, the lens power that brings distant objects to a sharp focus on the retina, plus the extra power needed for near. The assistant measures; the physician decides what to prescribe.

Refractive errors — in emmetropia, distance light focuses on the retina with accommodation relaxed. In myopia it focuses in front of the retina (minus lenses correct it); in hyperopia, behind it (plus lenses). In astigmatism the eye's two principal meridians focus at different points (a cylinder corrects it), and in presbyopia the aging crystalline lens can no longer focus up close (a plus add corrects it). A young eye can hide part of its hyperopia by accommodating, so nearly every step of a refraction works to keep accommodation relaxed.

Writing the result — for each eye, right first: sphere, cylinder, and axis, then the add. OD −1.25 −0.50 × 175 means a −1.25 D sphere combined with a −0.50 D cylinder whose axis lies at 175°. Axes run from 1 to 180 and are charted with three digits, so 090 rather than 90; DS means sphere only. Plus- and minus-cylinder forms describe the same lens. The spherical equivalent is the sphere plus half the cylinder; transposition and worked examples are in the Optics and Spectacles guide.

Retinoscopy — objective estimation of refractive error by neutralizing the retinal reflex ("with" motion → add plus; "against" motion → add minus). Objective means the patient gives no answers, so it suits young children and anyone who cannot respond reliably.

Fixation — the patient looks at a distant target beside your head, never at the light, so accommodation stays relaxed.

Neutrality — lenses are added until the reflex fills the pupil and stops moving as the streak sweeps. The far point is where the relaxed eye is focused; the reflex coaches you along the way:

Astigmatism — align the streak with each principal meridian and sweep perpendicular to it. Each meridian neutralizes at its own power; the difference between the two is the cylinder.

Working distance — at the end, subtract the working-distance sphere (67 cm → 1.50 D; 50 cm → 2.00 D): gross finding − working distance = net refraction. The correction is 1 divided by the working distance in meters, and it comes off the sphere only; cylinder and axis do not change.

The R lens — many phoropters have a retinoscopy lens marked R, usually +1.50 D for 67 cm. With it in place the dial already reads net; subtracting again leaves the patient 1.50 D too minus.

Autorefractors — fast objective starting points, but accommodation in young patients (instrument myopia) and media opacities in older ones mean the numbers are a beginning, not an endpoint.

Manifest or cycloplegic — manifest ('dry') shows the everyday eye; cycloplegic shows the full error.

Cycloplegic ('wet') refractions — for children and anyone whose accommodation hides latent hyperopia or won't hold still. Presbyopic eyes and pseudophakic eyes (those with a lens implant) have little accommodation left to hide anything. The physician compares the two results to decide what to prescribe.

The drops — cyclopentolate is the usual drop for a child's cycloplegic refraction; atropine is the strongest and longest-lasting (days), kept for specific cases. Tropicamide is mainly a dilating drop with only mild cycloplegic effect, and phenylephrine dilates with no cycloplegia at all, so a wide pupil alone does not prove accommodation is relaxed. All four dilate, so they go in only on the physician's order or protocol, after acuity and pupils are recorded and the patient is screened for a shallow chamber (see the Pharmacology guide, including atropine's risks in children). Wait the interval the protocol sets; record the drop and the time it went in.

Subjective refinement starts from the retinoscopy, autorefraction, or current glasses, refines each eye in turn, then balances the two.

1. Fog first — extra plus blurs the chart so accommodation has nothing to grab; reduce fog stepwise toward best acuity.

2. Bracket the sphere — "which is better, one or two?" in 0.25 steps, honoring the golden rule: maximum plus (least minus) for best acuity. Young eyes accommodate through excess minus and read the chart happily — then come back with headaches. Over-minus is the classic asthenopia (eye strain) story. Give more minus only when the patient reads more letters with it; smaller and blacker is not clearer.

3. Cylinder with the Jackson cross cylinder (JCC) — the JCC is a lens with equal plus and minus cylinder at right angles, flipped to show two views. AXIS before POWER (power measured off-axis reads falsely low): straddle the working axis, follow the preferred flip in shrinking steps until both flips look alike; then align the JCC with the correcting cylinder and bracket power, holding spherical equivalent as trained. In minus-cylinder form, add +0.25 D of sphere for each −0.50 D of cylinder added. Recheck the sphere once the cylinder is set.

4. Duochrome check — RAM-GAP: red clearer, add minus; green clearer, add plus. Red light focuses slightly behind green, so a sharper red side means the focus sits a little in front of the retina. The aim is equal clarity. Color-deficient patients can do it, because they judge sharpness, not color.

5. Binocular balance — equalize accommodative effort between the eyes (alternate fog or prism dissociation). Equal effort, not identical numbers, is the goal. It needs two eyes with similar acuity; when one is much weaker, skip the balance and chart why.

6. Presbyopic add — start from the age-based tentative add (~+1.00 early 40s to ~+2.50 by 60), refine at the patient's real working distance. Keep the full distance correction in place and set the phoropter to the near pupillary distance (PD). An add that is too strong makes print clear only when it is held too close.

Phoropter or trial frame — the phoropter holds the spheres, cylinders, and cross cylinder in one instrument; set it to the patient's PD and keep it level, since a tilt shifts every cylinder axis. A trial frame, worn by the patient with loose lenses, suits very strong prescriptions, small children, a head turn that steadies nystagmus, eccentric viewing (looking past a central blind spot), and letting a patient walk around in a big change before glasses are ordered.

High prescriptions — beyond ±4.00 D, record the vertex distance; effective power changes with lens position, and contact-lens conversion depends on it (see the Optics and Spectacles guide).

Pinhole — acuity that improves through a pinhole points to uncorrected refractive error (see the Visual Assessment guide). It also sets a target: if the final lenses do worse than the pinhole did, recheck the refraction, and report a gap that stays; an irregular cornea is one cause glasses cannot fix. If neither pinhole nor lenses help, the loss is probably not refractive, and the physician needs to know.

Before recording — confirm the best distance acuity each eye reaches with the final lenses, right first, then both together. Compare the result with the current glasses (read on the lensometer) and flag a large change for the physician. Record each eye's sphere, cylinder, and axis with that acuity; the add with its near acuity; the vertex distance for strong prescriptions; and whether the refraction was manifest or cycloplegic, with the drop used.

Be ready to find a spherical equivalent, neutralize with and against motion, handle the working distance, tell manifest from cycloplegic and who needs which drop, explain fogging and maximum plus, order the cross-cylinder steps, apply RAM-GAP, state the goal of binocular balance, estimate an add from age, and say when vertex distance is recorded.