Ametropic Eye

Here you can learn more about the structure of the eye, the various visual defects that exist, and the changes that occur to vision from about the age of 40 onwards.

1. Human Eye

Horizontal section through a human eye

Human Eye
Horizontal section through a human eye

2. Emmetropia

Optics and the eye, Emmetropic Eye
Emmetropic eye

In distant accommodation an emmetropic eye images an infinitely distant object point sharply on the retina, i. e. the focal point lies exactly on the retina.
The length (approx. 24 mm) and refractive power (Feye = +59 D) of the eye accommodated for distance vision are exactly matched to each other. The far point lies at infinity.

Interactive Demo: Eye with normal vision

Far Point MR
The eye can adjust to various distances by changing the refractive power of the crystalline lens (accommodation). When the lens displays the lowest possible refracting power, i. e. the longest possible distance setting, the term "distant accommodation" is used.
The far point MR is the object point which is sharply imaged on the retina in distant accommodation.

A lens is fully correcting if its image-side focal point F’ coincides with the far point MR.

3. Ametropia

Ametropic Eye
Ametropic Eye

If there is any disproportion between the refractive power and the length of the eye when it is accommodated for distance vision, the eye is ametropic. The far point does not then lie at infinity, but at a finite distance in front of or behind the eye.

A distinction is made between myopia (short-sightedness) and hyperopia or hypermetropia (far-sightedness). In both cases spherical or astigmatic ametropia (astigmatism) may be present.

Interactive Demo: Astigmatic eye

Spherical ametropia
Ametropia is classed as spherical if the refractive power of the eye is the same in all meridians. The focal point of the ametropic eye accommodated for distance vision lies either in front of or behind the retina.
A lens with a spherical power is required to correct this condition.

Astigmatic ametropia
Ametropia is classed as astigmatic if the eye displays two different refractive powers in two planes (principle meridians) perpendicular to each other.
The focal lines and the circle of least confusion lying between them can both be either in front of or both behind the retina. One focal line can also be in front of the retina and one behind it. It is also possible for one of the two focal lines to lie exactly on the retina.
A lens with an astigmatic power is required to correct this condition. The eye and the spectacle lens then together form an optical system whose focal point lies on the retina.

Ametropia
Eye with astigmatic ametropia

4. Myopia (short-sightedness)

Myopic Eye
top: myopic eye bottom: myopic eye with correcting minus lens

In the myopic eye accommodated for distance the focal point lies in the interior of the eye in front of the retina, with the result that visual acuity is reduced for distance vision.
To permit sharp distance vision again, the excessive refractive power must be corrected by a minus lens in such a way that the focal point of the lens/eye system once again lies on the retina. In myopia the far point MR lies at a finite distance in front of the eye.

Interactive demo: Short-sighted eye

Axial and refractive ametropia

If the refractive power of an ametropic eye corresponds to the refractive power of the schematic eye but its length is different, the term "axial ametropia" is used.
If the refractive power of the optical system deviates from the schematic eye while its length is normal, the term "refractive ametropia" is used.
Most types of ametropia are neither axial nor refractive, but are mixtures of the two. During refraction, it is not possible to determine what type of ametropia is present. What is important is the extent of ametropia, not its cause.

 

Hyperopia  

Myopia

Axial Ametropia

eye too short

eye too long

Refractive ametropia

refractive power too low

refractive power to high

5. Hyperopia (far-sightedness)

In a hyperopic eye accommodated for distance the focal point lies behind the retina. By increasing the refractive power of the crystalline lens (accommodation), hyperopia can be totally or partly corrected. Therefore, visual acuity for distant objects is not necessarily reduced, but the required accommodation may lead to eye strain.
To ensure that sharp distance vision is also possible when the eye is accommodated for distance, the insufficient refractive power must be corrected by a plus lens in such a way that the focal point of the lens/eye system lies on the retina. In hyperopia the far point MR lies behind the retina.

Hyperopic Eye
top: hyperopic eye bottom: hyperopic eye with correcting plus lens

Interactive Demo: Long-sighted eye

Axial and refractive ametropia

If the refractive power of an ametropic eye corresponds to the refractive power of the schematic eye but its length is different, the term "axial ametropia" is used.
If the refractive power of the optical system deviates from the schematic eye while its length is normal, the term "refractive ametropia" is used.
Most types of ametropia are neither axial nor refractive, but are mixtures of the two. During refraction, it is not possible to determine what type of ametropia is present. What is important is the extent of ametropia, not its cause.

 

Hyperopia  

Myopia

Axial Ametropia

eye too short

eye too long

Refractive ametropia

refractive power too low

refractive power to high

6. Presbyopia

Presbyopia
top: presbyopic eye in near vision bottom: presbyopic eye with correction plus lens in near vision

To be able to see close objects sharply, the refractive power of the eye must be increased compared to the refractive power required for distance vision. This is performed by accommodation, in which primarily the front surface of the crystalline lens increases its curvature.

With increasing age, however, the elasticity of the crystalline lens and hence the ability to accommodate decrease. Although this process already begins when we are young, it does not become noticeable until we reach the age of approx. 40 when our ability to accommodate has decreased to such an extent that focusing at close range is no longer possible. We have to hold a newspaper or book at arm’s length to read it, and soon even our arms are not long enough!

Interactive Demo: Vision after 40

Presbyopia with correcting lens

To be able to focus at close range again, the missing accommodative power of the presbyopic eye must be replaced by a spectacle lens. For the emmetropic presbyope, this is always a lens with a plus power. The following generally applies: the lens required for near vision has a higher power in the plus direction than the correction needed for distance vision.

Strictly speaking, presbyopia is not a type of ametropia, but is the result of a normal physiological development which occurs in every eye. The extent of presbyopia increases as we get older.