WHAT IS VISION?
A simple question it seems. However vision, the bodies ability to use the light reflected from an object and turn it into a representation of the object itself, is perhaps the most amazing process we know.

When you look at an object, say another person, you see that person as a solid 3 dimensional thing in the space around you. Actually all you are experiencing is a succession of electromagnetic waves of varying strengths passing into your eyes and causing a chemical change in cells at the back of your eyes.

If you want to delve deeper into how your visual system works then please click here. For the rest all that needs to be said is that the eye focuses light onto the retina and it is this image that is "seen" by the brain.

The accuracy of the focus determines whether you need spectacles or not.  When we are talking about vision we usually talk about distance vision and reading vision.
Distance vision
When the eye is at rest it should focus light from a distant object onto the retina. When it does it is called emmetropia and no extra focusing is required.
If it does not then the person sees objects blurred unless the eye can focus itself or a lens is placed in front of it to focus the image accurately. The strength of this lens is called the eyes Distance Prescription. 
If light gets focused too strongly then the light comes to a focus in front of the retina. The image is blurred again by the time it reaches the retina. The eye is then called short sighted (myopia) because close objects are often focused better. Eyes.jpg (40442 bytes) To correct a myopic eye we need to use a negative lens to effectively weaken the power of the eye.
If the light is not bent enough and does not come to a focus then the eye is long sighted (hypermetropia) because the problem is even worse for close objects. Whereas to correct a hypermetropic eye we use a positive lens to strengthen the focus.
If light is focused by different amounts depending on the orientation then the eye has an error called Astigmatism. The eye can still be long or short sighted. For astigmatism we use a toric lens with different powers in different meridians.