By: Carmen Willings
Visual field refers to how great an area a person can see and is measured, in degrees, as an angle. If a person with normal vision looks straight ahead, he should be able to see nearly all of the objects in a half-circle (approximately 160-180 degrees, with an equal area perceived on each side of the nose and 120 degrees on the vertical*). The central one-third of the visual field is seen by both eyes. A visual deficit may be a central field loss, or may occur elsewhere in the visual field. A person is said to have low vision if they can only see a 40-20 degree field, or less, in their best eye.
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Central vision (or macula vision), the "what" system, provides color discrimination and allows critical or sharp (exact) seeing tasks. Reading, whether at close or at a distance is a task that requires central vision. This is the vision which is improved when corrective lenses are prescribed. When the macula is not developed sufficiently (as in total cataract) or is deteriorated (as in macula degeneration) corrective lenses are of no value. Seldom can acuity be better than 20/200 if macula or central vision is lost. With this problem a student may wear no glasses, appear to see normally and not be able to do any critical visual task. The following conditions typically affect the central visual field: Macular Degeneration, Cone Dystrophies, and Central Scotomas.
Peripheral vision, the "where" system, provides awareness of movement and serves in dim light and is vital for mobility.
The following conditions typically affect the peripheral visual field: Chorioretinitis, Glaucoma, Retinal Detachment, and Retinitis Pigmentosa.
Some visual conditions can affect both combined central and peripheral field impairments: Coloboma, Optic Nerve Disorders and Diseases, Optic Atrophy, Optic Nerve Hypoplasia and Septo-Optic Dysplasia, Hemianopsia, and Strokes.
Types of Field Loss
There are different types of visual field loss.
If a student has scotomas or blind spots, the student could have an eccentric view and need to direct their gaze differently (ex. looking to the right or left or above the object being viewed) and will appear to not be looking directly at an object.
*Ward, Marjorie E., Foundations of Low Vision: Clinical and Functional Perspectives. AFB Press 2004. p. 73