Subjective refraction refines an objective starting point (retinoscopy, autorefraction or the current spectacles) using the patient's responses
The order: best sphere → cylinder axis → cylinder power → recheck sphere → binocular balance → near add
Each eye is tested with the other occluded (or fogged), in a trial frame or phoropter at a known vertex distance

## Best sphere

Aim: the maximum plus (or minimum minus) for the best visual acuity, with accommodation relaxed
Fogging: start with enough extra plus (about +1.00D) to blur the vision, which makes the eye artificially myopic and relaxes accommodation. Reduce the plus in 0.25D steps until the acuity stops improving
Signs of over-minus: letters look smaller and darker rather than clearer, and the patient accommodates to cope. Each extra 0.25D of minus should buy about one more line; if not, do not give it
Plus/minus 0.25D "which is clearer?" checks: if the two look the same, give the more plus

## The duochrome test

Black letters on a red and a green background. It relies on chromatic aberration: the eye refracts green (shorter wavelength) more than red, so green focuses in front of red, about 0.50D apart (see Aberrations)
"RAM GAP": if the letters on red are clearer, add minus; if the letters on green are clearer, add plus

Red clearer: the foci lie in front of the retina (over-plussed, or under-corrected myopia), with red the closer to the retina
Green clearer: the foci lie behind the retina (over-minused, or uncorrected hypermetropia)

The end point is equal clarity, or just red clearer, which avoids over-minus
It works in colour-deficient patients, because it depends on focus, not on colour discrimination
It is less reliable when acuity is poor, and older patients with yellowed lenses tend to favour red

## The Jackson cross-cylinder

A cross-cylinder has equal and opposite powers in meridians 90° apart, e.g. +0.25 / −0.50 × 90, which is ±0.25D. Its spherical equivalent is zero, so it does not move the circle of least confusion (see Astigmatic Lenses)
The handle lies 45° from both principal meridians. Twirling the handle flips the plus and minus axes
Use ±0.25D for good acuity, ±0.50D or more for poor acuity
The sphere must be correct first, so that the circle of least confusion is on the retina. Use a round target one or two lines above the best acuity

Step	Cross-cylinder position	Response
1. Axis (always first)	Handle along the trial cylinder axis, so the cross-cylinder axes straddle it at 45°	Rotate the trial minus cylinder axis towards the cross-cylinder's minus axis in the clearer position (plus towards plus for plus cylinders). Repeat and bracket until both flips look equal
2. Power	The cross-cylinder axes along the trial cylinder axis and at 90° to it	If clearer with the cross-cylinder minus axis on the trial axis, add −0.25D cylinder; if with the plus axis there, reduce the cylinder. Repeat until equal

Keep the spherical equivalent constant: for every −0.50D of cylinder added, add +0.25D of sphere
Axis steps: large (about 15°) for low cylinders and small (about 5°) for high ones, because an axis error matters more with a high cylinder
Finding cylinder when none is expected: check with the cross-cylinder axes at 90/180, then at 45/135
Worked example: trial lens −1.00 / −1.00 × 180. In the power check, the patient prefers the cross-cylinder plus axis along 180, so reduce the cylinder (using a ±0.50 cross-cylinder) to −0.50D and change the sphere to −1.25D (spherical equivalent stays −1.50D)

## The astigmatic fan (clock dial)

An alternative method for finding the cylinder axis. Fog the eye so both focal lines are in front of the retina, then ask which line on the fan looks darkest and sharpest
Rule of 30: multiply the lower clock-hour number of the darkest line by 30 to get the minus cylinder axis

Worked example: the 12–6 o'clock (vertical) line is darkest. 6 × 30 = 180, so the correcting cylinder is minus at 180 (with-the-rule astigmatism)

Add minus cylinder at that axis until all the lines look equal, then reduce the fog

## Binocular balancing

Prism dissociation: about 3Δ base up before one eye and 3Δ base down before the other separate the two images vertically. With both eyes fogged by +0.75D, add plus to whichever image is clearer until they are equal
Alternate occlusion with fogging
Humphriss immediate contrast: fog one eye by about +0.75D (it still contributes to fusion) and refine the other
Polarised or septum tests: each eye sees part of the chart

Finish with a binocular maximum plus for best acuity. Binocular acuity is usually a little better than monocular

Pinhole: if acuity improves with a pinhole, the reduced vision is at least partly refractive (see Light and Physical Optics)
Near add and the range of clear vision are tested last, at the patient's working distance (see Accommodation and Presbyopia)
Always compare the result with the current spectacles before prescribing a large change (see Prescribing Spectacles)