Vision Problems

The Normal Eye

normal eye vision

The eye is like a camera.  Both the camera and the eye have an opening for light to enter, a lens system to correctly focus the light and a screen upon which the image is projected.

Rays of light coming from an object enter the eye through a  transparent dome at the front of the eye. This is the cornea, which is the first lens in the system. It begins the bending or refracting of the light rays back toward a point of focus on the retina. The retina is the back of the eye upon which the image is projected. It's like the film in a camera.

After passing through the cornea, the light rays travel through the pupil, which is the round black circle within the colored part or iris of the eye. The iris acts like a circular window shade, making the pupil larger or smaller as the need for light changes.

Right behind the pupil the light rays pass through the crystalline lens of the eye. It is a flexible lens and its shape can be altered by a circular muscle at its circumference. Its function is to make the small final adjustments in focusing.

Behind the pupil the hopefully correctly focused rays of light pass through a gel-like substance called the vitreous and arrive at the retina. The retina is a transparent membrane containing a million light sensitive receptors. Those receptors send their signal to the brain which analyzes the signals and “we see“.

There are many reasons why an eye may not function properly. The most common of these is when the light does not end up perfectly focused on the retina. These are called refractive errors.

Myopia - The Nearsighted Eye

myopia nearsighted eye vision

A nearsighted eye is often thought of as weak, but in fact the optical system is really too strong. The light comes to a focus too soon, before it arrives at the retina so the image created at the retina is blurred. As a result distance vision, without glasses or contacts, is always blurred. Vision may be good at closer distances. Click images to enlarge them.

Hyperopia - The Farsighted Eye

farsighted hyperopia eye visionhyperopia farsighted eye visionA farsighted eye does not focus the light rays soon enough so the image of a distant object arriving at the retina is blurred. If the crystalline lens can change the focusing error enough vision will be clear at distance. If it can't vision will be blurred at distance. Vision at near, when farsighted, requires more focusing effort than distance. If the crystalline lens can change focus enough both far and near vision will be good. If it can't vision is worse at near than at far. Click images to enlarge them.

Astigmatism - The Astigmatic Eye

astigmatism. astigmatic visionThe astigmatic eye is unable to focus the entire image on the retina because of the "egg-shaped" curve of the cornea on the front of the eye. Rays entering the "North-South" curve of the cornea may focus correctly on the retina while those rays entering the "East-West" curve may not. Most people have a small amount of astigmatism, which doesn't really effect vision significantly. If they have more than a small amount it does effect vision, usually both at far and near. Click images to enlarge them.

Presbyopia - Focusing problems in the 40's

presbyopia presbyopic visionAs we get older the eye's crystalline lens begins to lose some of its elasticity. In the 40's we all experience increased difficulty focusing. This is first noticeable when small print becomes blurred at reading distances. As this loss of elasticity continues, larger objects, at greater distances from the eye, also become blurred. The range of clear vision eventually extends beyond arm's length. If you are a senior you have already gone through the stress of presbyopia. Your prescription will probably not change too much for a few years until you start to develop cataracts. This again is a natural part of life which usually starts in the mid-60's. It's nothing to fear - just another obstacle to overcome. Click images to enlarge them.

See the section of this page about eye disease.