The eye is like a camera, in that it has an optical system (the tear film, cornea, aqueous humour, lens, and vitreous humours) and a light-responsive ‘film’ (the retina). The image of an object is in focus on the retina when the optical power and the length of the eye are equal. The length of the eye is fixed, but the power of the eye can be altered by a process called accommodation during which a muscle within the eyeball (the ciliary muscle) contracts, causing a change in shape of the lens, and a consequent change in optical power.
If a distant object is in focus on the retina when the ciliary muscle is relaxed, then the power of the optical system matches, or is appropriate for, its length. This condition is termed emmetropia.
If, however, there is a mismatch between the power of the eye’s optical system and the length of the eyeball then the eye is said to have a refractive error. This error can take the following forms:
Hypermetropia (also known as hyperopia, or long-sightedness) occurs when the power of the relaxed eye is too low for the length of the eyeball, and light from a distant object is then focused behind the retina (or it would be if it could get there!). A more detailed explanation, along with a discussion of the functional consequences of this refractive error, can be found in the section entitled hypermetropia.
Myopia, or short-sightedness, is the opposite of hypermetropia - the power of the eye is too great for the length, and so light from a distant object is focused before it reaches, i.e. in front of, the retina. A more detailed explanation, along with a discussion of the functional consequences of this refractive error, can be found in the section entitled myopia.
If a person is short sighted, myopic, they can do little about this, apart from wearing an optical correction (contact lenses or spectacles) or having surgery. Some people will screw up their faces - closing their lids - which will enable them to see detail clearer at the expense of muscular load. If a person is long-sighted, hypermetropic, they have an additional strategy because they may be able to increase the power of their eye by using their accommodation (see above).
Astigmatism. The eye
differs from the camera, optically, in that it is not necessarily radially
symmetrical. If one were to examine two perpendicular cross sections
of the eye (e.g. horizontal and vertical) the length of each would be identical,
as would the optical power if the eye had no astigmatism. For an
eye without astigmatism the front surface of the cornea is spherical.
However, for many people the cornea’s front surface is actually shaped
like a rugby ball, or an American football. As a consequence of the
different curvatures in the two perpendicular meridians (i.e. the long
and short axes of the rugby ball) the optical power in each meridian differs,
and the person is said to have astigmatism (or is said to have an astigmatic
eye). A more detailed explanation, along with a discussion of the functional
consequences of this refractive error, can be found in the section entitled
astigmatism.
The above descriptions relate to the eye looking at a DISTANT object. When a normal person looks at a NEAR object, they need to accommodate (increasing the power of the eye) to bring the image into focus - in the same way that the focus of a camera needs to be changed to maintain a clear picture when changing between objects which are at different distances.
Presbyopia is when the person can no longer accommodate sufficiently to achieve clear vision. This happens when the lens hardens (NOT because of changes to the muscle as is often stated). Presbyopia is not normally considered to be a ‘refractive error’ because it is a normal consequence of age, and because it is a problem of near vision rather than of distance vision. A more detailed explanation, along with a discussion of the functional consequences, can be found in the section entitled presbyopia.
Finally, blurred vision is explained further here in a Powerpoint presentation.