A refraction result is not always the best prescription. Prescribing balances clarity against comfort, adaptation and the patient's needs
Writing a prescription: sphere / cylinder × axis for each eye, the near or intermediate add, any prism with its base direction, and the vertex distance for powers above about ±4.00D

## Rules of thumb

Compare with the current spectacles. If the patient is happy and the change is small, consider not changing
Myopes: give the least minus for the best acuity. Over-minus causes asthenopia and makes near work harder
Hypermetropes: adults with latent hypermetropia may not tolerate the full cycloplegic result. Give the manifest result, or partial correction, and increase over time. Children with esotropia get the full cycloplegic correction
Cylinders: a large change in power or axis, especially oblique, causes distortion and is poorly tolerated, particularly by older patients

Reduce the change and keep the spherical equivalent: e.g. halve the change in cylinder and add half of the removed cylinder to the sphere

Children: prescribe to treat amblyopia, strabismus or symptoms, or for significant errors (e.g. hypermetropia above about +3.50D, astigmatism above about 1.50D, anisometropia above about 1D), not for small errors in a comfortable child
Anisometropia: consider image size and vertical prism (see Anisometropia and Aniseikonia)

## Vertex distance

The vertex distance (back of the lens to the cornea) changes the effective power of high lenses (see effectivity in Spherical Lenses)
Moving a lens away from the eye makes a plus lens more effective and a minus lens less effective
Worked example: a myope is refracted with −8.00D at 14 mm, but the new frame sits at 10 mm

The far point is 1 ÷ 8 = 12.5 cm in front of the trial lens, so 13.9 cm in front of the cornea
A lens at 10 mm is 12.9 cm from that far point: 1 ÷ 0.129 = −7.75D

Measure the vertex distance with a distometer, and state it on prescriptions over ±4.00D

## Centration and induced prism

The optical centres should lie in front of the pupils. Measure the interpupillary distance (PD) for distance and near; the near PD is about 3–4 mm less
Looking away from the optical centre induces prism by Prentice's rule: P = c × F (c in cm) (see Prisms)
Worked example: +6.00D lenses for a patient with a 64 mm PD, mounted with optical centres 70 mm apart

Each optical centre is 3 mm outside the pupil: 0.3 × 6 = 1.8Δ per eye
The eyes look nasal to the optical centres of plus lenses, so each lens gives base out prism: 3.6Δ base out in total, a convergence demand
For minus lenses the same error gives base in prism

Tolerance: horizontal prism is better tolerated than vertical. A vertical difference of more than about 1Δ between the eyes may cause diplopia or asthenopia
Prescribed prism can be produced by deliberate decentration when the lens is powerful enough: c = P ÷ F

Worked example: 2Δ base in from +5.00D lenses. c = 2 ÷ 5 = 0.4 cm, so each optical centre is decentred 4 mm in (for a plus lens, decentre towards the base)

Pantoscopic tilt: frames tilt the lower rim towards the cheek. Lower the optical centre by about 1 mm for every 2° of tilt, so the optical axis passes through the eye's centre of rotation

## Bifocals

A segment adds the near power below the distance portion. Types: round, flat-top (D-segment) and executive (Franklin), where the segment runs the full width of the lens
Image jump: when the eye crosses the top of the segment, the segment's prism appears suddenly and the image jumps up

Jump = add × distance from the segment top to the segment's optical centre (cm)
It depends only on the add and the segment type, not on the distance lens
Worked example: a 24 mm round segment (optical centre 12 mm below its top) with a +2.50D add: 1.2 × 2.5 = 3Δ base down at the top, so the image jumps up
Flat-top segments have their optical centre close to the top (about 3–5 mm): little jump. Executive segments have it on the dividing line: no jump

Image displacement: the total prism at the reading position, from the distance lens and the segment together. It shifts where the reading material appears

Hypermetropes: the plus distance lens gives base up at the reading point; a round segment's base down opposes it. A round segment minimises displacement
Myopes: the minus distance lens gives base down at the reading point; a flat-top segment's base up opposes it. A flat-top segment minimises displacement
Displacement matters more to most patients than jump

Summary: hypermetropes do best with round segments, myopes with flat-top segments, and the executive bifocal gives no jump. Executive bifocals are also used for children with convergence excess esotropia, with the segment line bisecting the pupil.

## Progressive addition lenses

The power increases smoothly from distance through an intermediate corridor to near, with no visible line or jump
Unwanted peripheral astigmatism on either side of the corridor is unavoidable (Minkwitz theorem): the rate of change of astigmatism across the corridor is twice the rate of change of power along it. Higher adds and shorter corridors give more distortion
Fit with the fitting cross at the centre of the pupil in the primary position; the near zone is inset for convergence
Problems: swimming on head movement, narrow intermediate and near fields, a need to lower the gaze to read. Occupational (office) lenses widen the intermediate zone

## Lens forms and materials

Best form lenses are meniscus, with curves chosen to minimise oblique astigmatism (see Aberrations). Aspheric surfaces allow flatter, thinner high-power lenses with less distortion

Material	Refractive index	Abbe number	Notes
Crown glass	1.523	59	Scratch resistant, heavy, can shatter
CR39	1.498	58	Light, good optics, the standard plastic
Trivex	1.53	about 45	Impact resistant, light, better optics than polycarbonate
Polycarbonate	1.586	about 30	The most impact resistant; for children, monocular patients and safety eyewear. Low Abbe number gives colour fringes off axis
High-index plastics	1.60–1.74	32–42	Thinner and lighter for high powers; more reflection, so need an anti-reflection coating

A lower Abbe number means more dispersion, so more transverse chromatic aberration when looking through the edge of the lens
Coatings and tints: anti-reflection, scratch-resistant, UV-blocking, photochromic (darkens with UV, so less in a car), polarising (see Light and Physical Optics)

Monocular patients and children should have impact-resistant lenses (polycarbonate or Trivex)
A patient who cannot adapt to new spectacles: check the prescription, the optical centres and PD, the vertex distance and tilt, the base curve (a change in base curve alters magnification and distortion), and the segment or progressive fitting heights