vision: [...] 2b (1): mode of seeing or conceiving; [...] 3a: the act or power of seeing: SIGHT; 3b: the special sense by which the qualities of an object [...] constituting its appearance are perceived and which is mediated by the eye; [...] 
As an input modality for information processing, vision plays the most important role. The complexity and sophistication of visual sense is to a large extent beyond our full comprehension but there is a large body of experimental knowledge about the properties of vision collected. Detailed review of all these properties would be voluminous , one can say that in general the responses span very considerable dynamic ranges. At the same time the amount information processed is cleverly reduced giving illusion of photographic registration of reality coinciding with fast and precise extraction of very complex information.
From the applications point of view the properties of vision which are basic and important are light intensity response, color response, temporal response, and spatial responses.
There are many different levels at which these properties can be studied. At the receptor level, there are retinal receptors sensitive to light intensity and color. However, these raw sensitivities have little in common with perceived light and color and the information form the receptors undergoes numerous transformations making that the visual system is rather sensitive to the changes in light intensity and tries to preserve color constancy in changing illumination. These effects can be quite elaborate, depending also on higher level aspects of visual perception and memory. While the sensitivities of the receptor system is very high, for practical purposes it suffices to assume that it covers 8-10 bit range of amplitudes for the light intensity and each of the primary colors.
The temporal response of the visual system is responsible for many effects like perception of light intensity changes, and rendering of motion. The response can also be very high in specific conditions but in practice it can be considered to be limited to a maximum of 100 Hz for very good motion rendering and few tens of Hz for light intensity change. This is dependent on the type of visual stimulation, distance, lighting conditions and it has as a direct consequence that the frequency of repetition of pictures in TV and cinema is about 50 Hz but this is insufficent for computer displays which require 70-80 Hz or more to eliminate picture flickering effects.
Spatial response of visual system deals with the problems of visual resolution, width of the field of view, and spatial vision. There is a direct and strong impact of these factors on the visual perception. While in normal scenes the resolution to details needs not to be very high (twice the normal TV resolution is considered ``high definiton''), in specific situations the eye is very sensitive to the resolution and this is the reason while magazine printing might require a hundred times higher resoltuion than TV. A very important perceptual factor is the width of the field of view. While the center visual field which brings most information is essential, there is a much wider peripheral vision system which has to be activated in order ot increase the perceptual involvement (cinema vs. TV effect). On top of this there is a sophisticated spatial vision system which is partially based on binocular vision and partially on spatial feature extraction from monocular images.
The full visual effect coming from the fusion of visual responses to the different stimulations is rich and integrated to provide optimum performance for very complex scenes. Usually the optimum performance means extremely quick and efficient detection, processing, and recognition of patterns and parameters of the visual scenes.