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The field of view is the part of the observable world that is seen at any given moment.
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", 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.
Different animals have different fields of view, depending on the placement of the eyes. Humans have a 180-degree forward-facing field of view, while some birds have a complete 360-degree field of view. In addition the vertical range of the field of view may vary.
The range of visual abilities is not uniform across a field of view, and varies from animal to animal. For example, binocular vision, which is important for depth perception, only covers 140 degrees of the field of vision in humans; the remaining peripheral 40 degrees have no binocular vision (because of the lack of overlap in the images from either eye for those parts of the field of view). The aforementioned birds would have a scant 10 or 20 degrees of binocular vision.
Similarly, color vision and the ability to perceive shape and motion vary across the field of view; in humans the former is concentrated in the center of the visual field, while the latter tends to be much stronger in the periphery. This is due to the much higher concentration of color-sensitive cone cells in the fovea, the central region of the retina, as compared to the higher concentration of motion-sensitive rod cells in the periphery. Since cone cells require considerably brighter light sources to be activated, the result of this distribution is that peripheral vision is much stronger at night relative to binocular vision.
Field of View - Astronomy
The term field of view in astronomy refers to the amount of the sky that can be viewed at one time through an optical device such as binoculars or a telescope. It is expressed in degrees of angular measurement, and depends on the apparent field of view of the particular optical device, and the magnification used. The field of view of binoculars typically is 8 degrees or so. Higher magnification binoculars will have a narrower field of view - perhaps 5 degrees - while lower magnification or extra wide-angle binoculars may have a field of view of 10 degrees or more.
Consider an amateur-sized telescope, which may use a parabolic mirror of, say, 2500 mm (~100 in.) focal length. If one chooses a typical medium-power eyepiece of, say, 25 mm focal length, the magnification of the mirror-eyepiece combination will be 2500 mm / 25 mm = 100 power (or magnification).
If, as is typical, the eyepiece has an apparent field of view, when one looks into it, of, say, 50 degrees, then the actual field of view of the eyepiece on that particular telescope will be 50 degrees / 100 power = 0.5 degrees. This means that, when one looks through the eyepiece mounted on this particular telescope, one will see 0.5 degrees of the sky. This is about the apparent diameter of the Moon or Sun as seen from Earth.
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