The field of vision is the entire area an individual can see without moving their head or eyes. This total visible space is a composite image created by the brain from light entering both eyes. The integrity of this visual field is fundamental for everyday functions, allowing for safe navigation, judging distances, and maintaining spatial awareness for activities like driving. A consistent, wide field of vision ensures that an individual remains connected to their surroundings, which is why monitoring its health is an important part of routine eye care.
Defining the Field of Vision
The full extent of the visual field is measured in degrees, forming an irregular oval shape that is significantly wider than it is tall. For a single eye, the field extends approximately 100 to 110 degrees temporally (outward) from the center. It is restricted on the nasal (inner) side, typically reaching only 60 degrees due to obstruction from the nose and brow ridge. Vertically, the monocular field spans about 70 degrees downward and 60 degrees upward. When both eyes are open, the two monocular fields overlap significantly, creating the binocular visual field, which can span up to 200 degrees horizontally. This central overlap provides depth perception (stereopsis), which is crucial for accurately judging the spatial relationship between objects.
The Two Primary Components
The visual field is functionally divided into two distinct zones: a small, high-definition central region and a motion-sensitive peripheral area. Central vision is served by the macula, a small spot on the retina densely packed with cone photoreceptors. This concentration of cones allows for the highest visual acuity, enabling sharp, detailed tasks like reading, recognizing faces, and distinguishing colors.
The peripheral field surrounds the center and is dominated by rod photoreceptors in the outer retina. Rods are highly sensitive to movement and function well in low-light conditions, providing poor resolution and minimal color information. Peripheral vision is essential for detecting movement and alerting a person to potential hazards, while central vision provides the sharp focus once the eyes are directed.
How Field of Vision is Measured
The medical procedure used to systematically test and map the visual field is called perimetry. This assessment measures light sensitivity across the entire visual field to identify areas of reduced or absent vision.
A quick, qualitative screening method is the confrontation visual field test, often performed during a standard eye examination. The examiner compares their own peripheral vision to the patient’s by moving fingers in the four quadrants while the patient fixates on the examiner’s nose.
For a precise and detailed analysis, automated perimetry is used, typically with a Humphrey Field Analyzer. The patient places their head in a bowl-shaped instrument and stares at a fixed central point. The machine flashes small, dim lights at various points, and the patient presses a button every time a light is seen. This static perimetry method generates a detailed map of retinal sensitivity, allowing specialists to detect subtle, early-stage blind spots.
Common Causes of Vision Field Loss
Loss of the visual field, known as a visual field defect, results from damage anywhere along the visual pathway, from the eye to the brain. One of the most frequent ocular causes is glaucoma, a progressive condition often linked to high internal eye pressure that damages the optic nerve. Glaucoma typically causes gradual, irreversible loss of peripheral vision, which can eventually lead to a constricted view known as tunnel vision.
Neurological events, such as a stroke or the presence of a brain tumor, can cause predictable patterns of vision loss affecting both eyes simultaneously. For example, a lesion in the visual processing centers can result in a homonymous hemianopia, where the same half of the visual field is lost in each eye. Damage to the optic nerve from conditions like optic neuritis can lead to a central or paracentral scotoma, a localized blind spot near the center of vision.
Retinal diseases also cause characteristic field loss patterns. Age-Related Macular Degeneration (AMD) destroys central vision. Retinitis Pigmentosa (RP) is an inherited disorder that initially damages peripheral rod photoreceptors, leading to progressive night blindness and a slow, concentric constriction of the visual field. Because many of these defects begin subtly and can be missed by the patient, regular perimetry testing is necessary for early detection and management.