A contact lens sits on the tear film, so it moves with the eye and its vertex distance is effectively zero
Optical consequences compared with spectacles

## The vertex effect

Contact lens power: F_c = F_s ÷ (1 − dF_s), where F_s is the spectacle power and d the vertex distance in metres (see effectivity in Spherical Lenses)

Myopes need less minus in contact lenses; hypermetropes and aphakes need more plus
Below about ±4.00D the difference is less than 0.25D and is usually ignored

Worked example: −6.00D spectacles at 12 mm. F_c = −6 ÷ (1 + 0.072) = −5.60D, so prescribe −5.50D
Worked example: +4.00D at 12 mm gives +4.20D, so +4.25D
For an astigmatic prescription, convert each principal meridian separately

## Rigid lenses and the tear lens

A rigid gas-permeable (RGP) lens keeps its own shape. The space between its back surface and the cornea fills with tears, which form a liquid lens
The tear lens power depends on the difference between the lens's back optic zone radius (BOZR) and the corneal curvature (K)

Fitted steeper than K: a plus tear lens, so the contact lens needs more minus
Fitted flatter than K: a minus tear lens, so the contact lens needs more plus
Mnemonic: SAM-FAP, steeper add minus, flatter add plus
Rule of thumb: 0.05 mm of radius difference ≈ 0.25D, so 0.1 mm ≈ 0.50D

Worked example: K = 7.80 mm (43.27D with the keratometric index 1.3375); BOZR 7.70 mm (43.83D). The lens is 0.1 mm steeper, so the tear lens is about +0.50D. If the refraction at the cornea is −3.00D, the lens power is −3.50D

Tears (n = 1.336) have nearly the same index as the cornea (1.376), so a spherical RGP lens replaces the toric anterior cornea with a spherical optical surface. It corrects corneal astigmatism without a cylinder
It does not correct lenticular (residual) astigmatism. Residual astigmatism = refractive astigmatism − corneal astigmatism
Worked example: refraction −2.00 / −2.50 × 180, keratometry shows 1.50D with the rule. A spherical RGP leaves about 1.00D of residual astigmatism, which would need a toric lens
RGP lenses are the optical treatment of choice for irregular astigmatism (keratoconus, scars, grafts). Scleral lenses vault the cornea entirely

## Soft and toric lenses

Soft (hydrogel or silicone hydrogel) lenses drape over the cornea and take its shape, so there is almost no tear lens. They do not correct corneal astigmatism
Astigmatism of about 0.75D or more needs a soft toric lens. It must be stabilised against rotation

Prism ballast (a heavier base down), dynamic stabilisation (thin zones above and below, held by the lids), and truncation
Lenses carry an orientation mark. If the mark settles rotated, adjust the ordered axis by the rotation: LARS, left add, right subtract (from the examiner's view)
Worked example: a lens ordered at axis 180 settles with its mark 10° to the examiner's left. Re-order at 180 + 10 = axis 10

Oxygen supply: the cornea depends on oxygen through the lens, described by Dk/t (permeability ÷ thickness). Silicone hydrogels have a much higher Dk than conventional hydrogels; low Dk/t causes hypoxia (neovascularisation, oedema)

## Accommodation and convergence

A myope must accommodate and converge more with contact lenses than with spectacles; a hypermetrope less

Worked example: reading at 33 cm from the spectacle plane (12 mm vertex distance). A −8.00D myope accommodates about 2.4D in spectacles and about 2.9D in contact lenses; a +8.00D hypermetrope about 3.6D in spectacles and 2.9D in contact lenses
So an early presbyopic myope who switches to contact lenses may notice near blur for the first time

Convergence: looking at near through spectacles, the eyes look nasal to the optical centres. Minus lenses give base in prism (less convergence needed); plus lenses give base out (more convergence needed). Contact lenses move with the eyes, so these effects disappear

## Magnification and field

Compared with spectacles, contact lenses make images larger for myopes and smaller for hypermetropes, because spectacle magnification depends on the lens's distance from the eye (see Magnification)
In unilateral aphakia, a contact lens gives about 7–10% magnification against 25–33% for spectacles, so binocular single vision is possible (see Anisometropia and Aniseikonia)
Contact lenses give a full field of view with no ring scotoma, no edge distortion, and no oblique astigmatism on looking through the periphery

Orthokeratology: a reverse-geometry RGP lens worn overnight flattens the central cornea, temporarily correcting low myopia (up to about −4.00D). It is also used to slow myopia progression in children
Monovision and multifocal contact lenses correct presbyopia (see Accommodation and Presbyopia)
Bandage contact lenses protect the cornea and do not need to correct refraction
Contact lens keratitis, especially Pseudomonas and Acanthamoeba, is linked to overnight wear and contact with water