Myopia

Strange as it seems, being nearsighted means you can’t see things far away. The usual problem stems from your eyeball being too long, or your cornea being too curved. When your eyeballs have this problem the focal points wind up in front of the retina where, when light reaches it, blurred images scatter.

Hyperopia

Farsighted means you can’t see things near. Why? The cornea does not have enough curve or you have a short eyeball. Tough luck. In this situation the focal point rests behind the retina. When light enters the retina, blurring again takes place because the images should have been focused before getting to the retina, not after.

Astigmatig

If you suffer astigmatism chances are your eyeballs have a football-like shape when they should be more spherical. Instead of focusing on one point, light entering the eye focuses on several and you get blurring images.

You can be both astigmatic and near-sighted or far-sighted, the latter requiring bi-focals. Most people address these problems with corrective lenses—glasses or contacts—but laser surgery can allow you the opportunity to see without glasses if you are nearsighted or astigmatic. One laser device has been approved for use to correct farsightedness, but less research exists on the results in farsighted people.

Presbyopia

Among the 40-and-older crowd, it’s a rare individual who doesn’t need some visual enhancements, even if they’re just off-the-rack reading glasses from the drug store. Farsighted folks (hyperopes) suddenly find that they need longer arms to read the newspaper or a restaurant menu. Nearsighted folks (myopes) may be removing their glasses more frequently to read things up close.

The increasing difficulty most of us experience as we age is due to a condition known as presbyopia. The effects of presbyopia make us increasingly farsighted as we get older.

Fields of Vision

Seeing an object and recognizing it for what it is involves image processing by cells in the retina and brain. Three type of cells are involved in this processing: simple, complex and hypercomplex. Simple cells respond best to a clearly defined slit of light (or dark) or to edges between dark and light areas, but the images have to be exact locations in the receptive field of the cell and in the proper orientation. Complex cells respond to slits and edges as well, but they generally react to any location or orientation of the image. The hypercomplex cells respond best to even more specific features such as corners, angles and lines of exact length, orientation and location. The area which encompasses the sight of both eyes is called the "field of vision". Capturing an object, whether standing still or moving, and converting it to nerve signals is only part of the process of seeing. In order for us to see, the brain must analyze the signals and draw meaningful information from them. Some nerve signals are passed from the retina to the temporal lobes of the brain, near the sides of the head, and these contribute to the processes of visual recognition and memory. Another section of the brain acts to control visual attention. Objects on the periphery of vision are detected and the head and eyes are instructed to turn and bring the object into focus inside the visual field. Another set of nerves carries signals to the cerebellum, the part of the brain responsible for muscular coordination. For full depth perception, the stereoscopic vision by two eyes is a necessity. Each eye views a scene from a slightly different angle, and the fusion of these two images gives a three-dimensional effect. With objects less than 200 feet away, vergence comes into play, so that each eye is angled in a little to focus directly on the object; the larger the object, the closer it is. When objects are moving, the brain knows that close objects pass faster than those farther away, and it can, therefore, compare the apparent speeds. Perfect 20/20 vision will not be enough to pass an eye test given to military pilots. It also involves "contrast sensitivity." One must be able, for example, to see a white cat walking in the snow.