Visual field and its effects
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the “spatial array of visual sensations available to observation in introspectionist psychological experiments” (J. Smythies ), while field of view “refers to the physical objects and light sources in the external world that impinge the retina”. In other words, field of view is everything that (at a given time) causes light to fall onto the retina. This input is processed by the visual system, which computes the visual field as the output.
The term is often used in optometry and ophthalmology, where a visual field test is used to determine whether the visual field is affected by diseases that cause local scotoma or a more extensive loss of vision or a reduction in sensitivity (threshold).
It is also important to note that it also matters whether you avert right or left. The most effective direction is that which places the object on the nasal side of the vision. So, for right-eyed observers it is best to shift to the right, and for left-eye observers it is best to shift to the left. Some people also claim that it is better to avert up instead of down. The best thing to do is practice and find the best location for one’s own eyes.
The normal human visual field extends to approximately 60 degrees nasally (toward the nose, or inward) in each eye, to 100 degrees temporally (away from the nose, or outwards), and approximately 60 degrees above and 75 below the horizontal meridian. In the United Kingdom, the minimum field requirement for driving is 60 degrees either side of the vertical meridian, and 20 degrees above and below horizontal. The macula corresponds to the central 13 degrees of the visual field; the fovea to the central 3 degrees.
The visual field is measured by perimetry. This may be kinetic, where points of light are moved inwards until the observer sees them, or static, where points of light are flashed onto a white screen and the observer is asked to press a button if he or she sees it. The most common perimeter used is the automated Humphrey Field Analyzer.
Patterns testing the central 24 degrees or 30 degrees of the visual field, are most commonly used. Most perimeters are also capable of testing the full field of vision.