A Wisdom Archive on Vision

The values indicated in the "sphere" and "cylinder" columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A +10 diopter lens would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus. Stacking lenses combines their strength. A +1 diopter lens combined wi ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Lens strength

To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired. This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of ...

Read more here: » Binocular vision: Encyclopedia II - Binocular vision - Disorders of binocular vision

Usually: the "spherical" component is the main correction the "cylindrical" component is "fine tuning." Depending on the optical setup is, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur. Whatever the setup, spherical lenses act equally in all meridians: they magnify, introduce blur, or correct blur the same amount in every direction. An ordinary magnifying glass is a kind of spherical lens. When a spherical lens acts as a magnifier, it magnifies equally in all meridians. Here, ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Spherical lenses and spherical correction

Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses. When a cylindrical lens acts as a magnifier, it magnifies only in one direction. For example, the magnifier shown magnifies letters only in height, not in width. Similarly when a cylindrical lens puts an optical system out of focus and introduces blur, it blurs only in one direction. This is the kind of blur that results from uncorrected astigmatism. The letters are smeared out directionally, as ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Cylindrical lenses and cylindrical correction

Some animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view. Other animals, usually predator animals, have their two eyes positioned on the front of their heads, thereby reducing field of view in favour of stereopsis. In such animals, the eyes usually move together. When the eyes move laterally, in ...

Read more here: » Binocular vision: Encyclopedia II - Binocular vision - Fields of view and eye movements

Once the fields of view overlap, there is a potential for confusion between the left and right eye's image of the same object. This can be dealt with in two ways: one image can be suppressed, so that only the other is seen, or the two images can be fused. If two images of a single object are seen, this is known as double vision or diplopia. Fusion of the images from the two eyes is considered to be separate from stereopsis for at least two reasons. First, some disorders of binocular vision, such as strabismus can spare fusion but abolish ste ...

Read more here: » Binocular vision: Encyclopedia II - Binocular vision - Singleness of vision

Spherical lenses just have a strength, such as +1.0D, or -2.5D. Astigmatism, however, causes a directional blur. Here are two examples of the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing. A cylindrical lens of the right strength can correct this kind of blur. The second example is a little bit more blurred, and needs a stronger cylindrical lens. But notice that in addition to being smeared more, the second example is smeared out in ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Axis

Blur may appear differently depending on the type of refractive error. The following are some examples of blurred images that may result from refractive errors: ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Blur

People whose eyes failed to align correctly could have no binocular vision. However, to an artist, this disability could have been a gift. In an article published on September 16, 2004 in The New England Journal of Medicine, Margaret S. Livingstone, professor of neurobiology of Harvard Medical School, suggested that Rembrandt, generally considered one of the greatest painters in European art history, suffered from this disabili ...

Read more here: » Binocular vision: Encyclopedia II - Binocular vision - Rembrandt as a person without binocular vision

The leftmost image above shows an eye chart as it might be seen by a person who needs no correction, or by a person who is wearing eyeglasses or contacts that properly correct any refractive errors he or she may have. The images labelled 1D, 2D, and 3D give a very rough impression of the degree of blur that might be seen by someone who has one, two, or three diopters of refractive error. For example, a nearsighted person who needs a -2.0 diopter corrective lens will see something like the 2D ima ...

Read more here: » Eyeglass prescription: Encyclopedia II - Eyeglass prescription - Amount of refractive error and degree of blur