Field of view
The field of view (also field of vision) is the angular extent of the observable world that is seen at any given moment.
Different animals have different fields of view, depending on the placement of the eyes. Humans have an almost 180-degree forward-facing field of view, while some birds have a complete or nearly-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.

# Conversions
Many optical instruments, particularly binoculars or spotting scopes, are advertised with their field of view specified in one of two ways: angular field of view, and linear field of view.  Angular field of view is typically specified in degrees, while linear field of view is a ratio of lengths.  For example, binoculars with a 5.8 degree (angular) field of view might be advertised as having a (linear) field of view of 305 feet per 1000 yards or 102mm per meter.  As long as the FOV is less than about 10 degrees or so, the following approximation formulas allow one to convert between linear and angular field of view.  Let A be the angular field of view in degrees.  Let L be the linear field of view in feet per 1000 yards.  Let M be the linear field of view in millimeters per meter.  Then:

# Astronomy
In astronomy the field of view is usually expressed as an angular area viewed by the instrument, in square degrees, or for higher magnification instruments, in 
square arc-minutes. For reference the Wide Field Channel on the Advanced Camera for Surveys on the Hubble Space Telescope has a field of view of 10 sq. arc-minutes, and the High Resolution Channel of the same instrument has a field of view of 0.15 sq. arc-minutes. Ground based survey telescopes have much wider fields of view. The photographic plates used by the UK Schmidt Telescope had a field of view of 30 sq. degrees. The 1.8m Pan-STARRS telescope, with the most advanced digital camera to date has a field of view of 7 sq. degrees. In the near infra-red WFCAM on UKIRT has a field of view of 0.2 sq. degrees and the forthcoming VISTA telescope will have a field of view of 0.6 sq. degrees. Until recently digital cameras could only cover a small field of view compared to photographic plates, although they beat photographic plates in quantum efficiency, linearity and dynamic range, as well as being much easier to process.