Near-Sighted, Far-Sighted or Astigmatism?
To get good vision the image of whatever object you are looking at must come into focus on the retina. For that to happen the power of the focusing system of the eye, composed of the cornea and lens, must be just right so the image falls exactly on the retina.
It is just like focusing a camera, with the retina being the film in the camera.
There are five terms that refer to the possible refractive conditions of the eye:
- Emmetropia: Good vision without glasses
- Myopia: Near-sighted
- Hyperopia Far-sighted
- Astigmatism: Curvature uneven
- Presbyopia: Need for reading glasse
Light from a distant object is focused exactly on the retina, making a sharp image. No glasses needed.
Light from a distant object is focused in front of the retina. If you get closer to the object the image moves backward, eventually falling on the retina. So, if you see better up close than far away you are “near-sighted.”
Correction is with a concave lens, which causes the light to diverge, moving the focus point backward.
Light from a distant object focuses behind the retina. Now as you approach the object it gets harder to focus. Since you see better at distance this is called “far-sighted.” If the image does not fall too far behind the retina the focusing muscles inside the eye can act on the lens to increase power and bring the image forward onto the retina. If the muscles have to work for extended periods they will tire.
Correction is with a convex lens, which converges light, thereby bringing the focus point forward.
Ideally the cornea and lens would be spherical in shape, like a basketball. However, many eyes do not have even curvature. Picture the shape of a football with a flatter curve on the long axis and a steeper curve on the short axis. Having more than one amount of curvature will cause the formed image to be spread out within the eye.
In the illustration the lens is meant to show a flatter curve vertically and a steeper curve horizontally, like a football standing on its end. The flatter curve bends light less, so that part of the image, the vertical part (in red), focuses further back. The steeper, horizontal part (in blue), bends light more, so the image forms further forward.
Correction is with an “astigmatic” lens that accounts for the different curves, bringing the whole image to focus in one plane. Since the astigmatism, or football, can be oriented in any direction, not just horizontal or vertical, on a glasses prescription the position is noted in degrees going clockwise from 12:00.
Accomodation is the process in which the eye changes focus from a distant object to a close one. (Like driving and text messaging at the same, which you know you should not be doing.) Three things happen in this process.
First, the focusing muscles act on the lens to increase its power. Second the eyes converge so they are both pointed at the same object. If they remained parallel, like at distance, they would be looking at different sides of a page. Third, the pupils constrict.
The illustration shows the eye looking at a letter E, representing a page of print held at reading distance. In the relaxed eye the image has fallen behind the retina. As with hyperopia, the focusing muscles can act on the lens to increase its focusing power and bring the image forward to focus on the retina.
Unfortunately, as the years go by the ability to crank in that focusing power decreases as the lens loses flexibility. Reading becomes progressively more difficult, until eventually reading glasses are needed.
Accomodation and Age
The above graph shows the decrease in accommodation (focusing power) that occurs with age.
(The units are diopters (D), a measure of lens power, equalling 1/focal distance in meters. 1 D focuses at 1 meter. 2 D focuses at ½ meter.)
In youth the lens is very flexible giving a lot of reserve to do close focusing. As flexibility decreases so does the focusing reserve and some time in the early to mid 40’s one can no longer comfortably focus up close. Correction is with reading glasses or bifocals. If you have a pair of bifocals you may have noticed the reading sections are set toward the nasal side of each lens. That is because your eyes converge when you look at objects up close.
See the Refraction section for how glasses power is determined and what options are available in glasses.