Emmetropia: with accommodation relaxed, parallel light from a distant object focuses on the retina
Ametropia: with accommodation relaxed, parallel light does not focus on the retina

Myopia: it focuses in front of the retina. The eye is too strong for its length
Hypermetropia (hyperopia): it focuses behind the retina. The eye is too weak for its length
Astigmatism: the power differs between meridians, so there is no single focus (see Astigmatism)

## The far point

The far point is the point conjugate with the fovea when accommodation is relaxed: the point that is in focus without any effort

Emmetropia: at infinity
Myopia: a real point in front of the eye. A −4.00D myope sees clearly out to 25 cm
Hypermetropia: a virtual point behind the eye, towards which converging light must be aimed

Correcting principle: a correcting lens forms an image of a distant object at the eye's far point. Its second focal point must coincide with the far point

Myopia is corrected with a minus (concave, diverging) lens; hypermetropia with a plus (convex, converging) lens
The ocular refraction (the error at the corneal plane) is 1 ÷ the far-point distance from the cornea
Worked example: a myope's far point is 20 cm in front of the cornea. Ocular refraction = 1 ÷ −0.20 = −5.00D. A spectacle lens 12 mm in front of the cornea must have its focal point 18.8 cm away: 1 ÷ −0.188 = −5.32D

The difference between the spectacle and ocular refraction grows with power (see effectivity in Spherical Lenses and Contact Lens Optics)

## Axial and refractive ametropia

Type	Myopia	Hypermetropia
Axial: abnormal length, normal power (the commonest)	Long eye. Each extra 1 mm of axial length gives about −3.00D	Short eye, e.g. nanophthalmos, microphthalmos
Curvature: steep or flat cornea or lens	Keratoconus, lenticonus, spherophakia	Cornea plana, post-myopic refractive surgery overcorrection
Index: change in refractive index	Nuclear sclerosis (index myopia, "second sight"), hyperglycaemia	Cortical changes with age; aphakia is extreme hypermetropia from loss of the lens
Position of the lens	Anterior lens movement: ciliary spasm, miotics, anterior lens subluxation	Posterior lens displacement

In the reduced eye (+60D, n = 1.336), 1D of ametropia corresponds to about 0.37 mm of axial length, or about 2.7D per mm, usually rounded to the rule of thumb of 3D per mm (see Optics of the Eye)
Axial or refractive type matters when choosing spectacles or contact lenses in anisometropia (see Magnification and Anisometropia and Aniseikonia)

## Myopia

Definitions (International Myopia Institute): myopia is a spherical equivalent of −0.50D or more with accommodation relaxed; high myopia is −6.00D or more. High myopia carries pathological risks
Symptoms: blurred distance vision with good near vision. Myopes may screw up their eyes (a stenopaeic effect)
Pathological (degenerative) myopia: posterior staphyloma, lacquer cracks, choroidal neovascularisation, myopic maculopathy, retinal detachment, glaucoma and early cataract
Myopia is rising worldwide, especially in East Asia. Risk factors are parental myopia, near work, education and little time outdoors
Slowing progression in children: low-dose atropine, myopia-control spectacle lenses (defocus incorporated multiple segments), dual-focus soft contact lenses and orthokeratology

## Hypermetropia

The young hypermetrope can overcome a low error by accommodating, which divides the hypermetropia into parts

Latent: masked by ciliary tone; revealed only by cycloplegia
Manifest: revealed without cycloplegia. It splits into facultative (overcome by accommodation) and absolute (cannot be overcome, so distance vision is blurred)
Total = latent + manifest. With age, accommodation falls: latent hypermetropia becomes facultative, and facultative becomes absolute

Symptoms: asthenopia and blurred near vision first, then blurred distance vision. Early presbyopia
Associations: accommodative esotropia (accommodative convergence), amblyopia, and angle closure (a short eye with a crowded anterior segment)
Worked example: a 25-year-old with +3.00D of hypermetropia must accommodate 3D for distance and 5.5D at 40 cm (3 + 2.5)

Babies are born hypermetropic (about +2.00D). Emmetropisation in the first years matches axial length to the eye's power
Children need a cycloplegic refraction to reveal latent hypermetropia, especially with esotropia
Cyclopentolate 1% is the usual agent; atropine gives the fullest cycloplegia, for example in accommodative esotropia

## Correcting ametropia

Spectacles: the simplest and safest correction. Higher powers bring magnification, field and aberration problems (see Prescribing Spectacles)
Contact lenses: a wider field and less change in image size. The power differs from spectacles at higher powers (see Contact Lens Optics)
Refractive surgery

Corneal: PRK, LASIK, SMILE. Myopic ablation flattens the central cornea; hypermetropic ablation steepens it
Lenticular: phakic IOLs for high myopia; refractive lens exchange

A sudden myopic shift in an adult suggests nuclear cataract, hyperglycaemia or drugs that cause ciliary body swelling (e.g. topiramate, which can cause bilateral angle closure)
A hypermetropic shift suggests a falling blood sugar after treatment of diabetes, or a lifted macula from fluid, tumour or a choroidal fold (the retina moves forward, which shortens the effective axial length)