Subjective Refraction Step by Step: Fogging, Best Sphere, Cross-Cylinder, Binocular Balance and Near Add

Last updated: November 2026

RPC Rounds for Optometrists · B.Optom study guide

Subjective refraction turns an objective starting point into the prescription the patient will actually wear. A consistent routine avoids the common errors: too much minus, the wrong axis, and an unbalanced pair of eyes. This is a standard monocular-then-binocular routine with trial frame or phoropter.

1. History and starting point

• Ask about symptoms, occupation, screen use, driving, previous glasses and how they suited, and general and eye health.
• Measure unaided and habitual vision at distance and near; read the current glasses with the lensometer.
• Start from retinoscopy (corrected for working distance: at 67 cm subtract +1.50 D), autorefraction or the old glasses.

2. Monocular refraction (fog first, then reduce)

1. Occlude the left eye. Fog the right eye with about +1.00 D over the starting sphere (vision about 6/18–6/24) to relax accommodation.
2. Reduce the plus in 0.25 D steps until the best vision is reached: maximum plus for best visual acuity (MPMVA). Each 0.25 D should improve vision by about a line; stop when extra minus only makes letters smaller and darker.
3. Check with the duochrome (red-green): letters equal or slightly clearer on red is the end point. Clearer on green means too much minus: add +0.25 D. Do not use it in poor colour vision; it works in most colour-deficient patients because it relies on chromatic aberration, but responses can be unreliable.

3. Astigmatism: Jackson cross-cylinder (JCC)

1. Use a round target one line larger than best acuity, with the circle of least confusion on the retina (spherical equivalent).
2. Axis first: place the JCC with its handle along the cylinder axis ("straddling"); flip and ask which is clearer. Move the axis towards the minus (red) mark of the preferred position: 15° steps for low cylinders, 5° for high, reversing and halving until both look equal.
3. Power next: align the JCC axes with the cylinder axis; if the minus position is preferred add −0.25 D cylinder, if plus is preferred reduce it. For every −0.50 D change in cylinder, add +0.25 D to the sphere to keep the spherical equivalent.
4. If there was no cylinder, check at 90° and 180°, then 45° and 135°, with a −0.25 D cylinder to discover small astigmatism.
5. Recheck the sphere (MPMVA) after the cylinder is found.

4. Binocular balance

• Fog both eyes with +0.75 to +1.00 D, then dissociate with vertical prisms (3Δ base up before one eye, 3Δ base down before the other) or polarised charts.
• Ask which line (top or bottom) is clearer; add +0.25 D to the clearer eye until equal. If they never balance, leave the dominant eye slightly clearer.
• Remove the prisms and reduce the fog binocularly to the best vision (binocular MPMVA). Binocular vision allows slightly more plus than monocular because accommodation relaxes.

5. Near add

• Start from an age-based add (about +1.00 at 45, +1.50 at 50, +2.00 at 55, +2.50 at 60 and over) or from half the amplitude kept in reserve.
• Refine with the patient's working distance using near charts, binocular cross-cylinder or the range of clear vision (the add should leave the working distance in the middle of the range).
• Do not over-plus: too strong an add shortens the range and causes complaints with computers.

6. Prescribing decisions

• Compare with the old glasses: avoid large changes in cylinder or axis in adults without symptoms; consider a partial change.
• Trial-frame the result and walk the patient round, especially with cylinder changes, high powers or first progressives.
• Record the vertex distance for prescriptions over ±4.00 D.

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Related: Jackson Cross-Cylinder: How to Refine Axis and Power, Duochrome Test: Red-Green Rule (RAM GAP) Explained, Retinoscopy Tips: With vs Against Movement and Working Distance, Refraction Calculators: Near Add, AC/A Ratio, Retinoscopy and Prism, How to Read Your Glasses Prescription: SPH, CYL, AXIS, ADD. Back to the optometry curriculum.

Key sources

• Elliott DB. Clinical Procedures in Primary Eye Care. 5th ed. Elsevier; 2020
• Benjamin WJ. Borish's Clinical Refraction. 2nd ed. Butterworth-Heinemann; 2006
• Grosvenor T. Primary Care Optometry. 5th ed. Butterworth-Heinemann; 2007