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The human eye exhibits remarkable performance in its own right, but when combined with the brain the result is even more impressive. Numerous detailed studies of the performance of the human eye and brain combination have been undertaken over the decades, and it is probably fair to say that most have give comparable but nevertheless slightly different results. In the age of digital imaging, comparison of the visual acuity of the eye and brain combination with the resolution of digital cameras and sensors may be of some interest.

When we look at a particular scene, our eyes do not capture a single image in the manner of a camera. Instead they provide a stream of information which the brain uses to build a broader and more detailed image than might otherwise be the case. Our eyes move rapidly through relatively small angles as we view a scene, so gathering more information than would be acquired by a single glance. The brain then combines the various sets of slightly different information to assemble a complete picture. This is a very sophisticated process by comparison with which all cameras must be considered very basic. Human beings also have two eyes that provide marginally different inputs because of their physical separation. However, the brain is also able to use this additional information to enhance the resolution of the image assembled by the brain. Consequently, the eye and brain combination is capable of assembling an image of much higher resolution than might seem possible based purely upon the number of receptors present in the retina. Direct comparisons with a digital camera are therefore extremely difficult. Nevertheless, some general assumptions can be made.

The critical visual angle, that is the minimum angle subtended by an object which the eye can resolve as a non-point source, is typically quoted as approximately 0.7 minutes of arc. However this value changes with brightness and contrast, and also varies for each particular eye. The visual angle of 0.7 minutes of arc is normally achievable only in normal daylight or good office or shop lighting. In a digital camera, a minimum of two pixels are required to resolve an object as being other than a point so, using the figure of 0.7 minutes of arc, each pixel must be 0.35 minutes of arc or less at the limit of visual acuity.

If a human being views a scene with an angle of view of approximately 75 degrees, both horizontally and vertically, the number of pixels required by a camera to produce a similar resolution would be (75 degrees x 60 minutes of arc per degree x 1/0.35)2 = approximately 165 megapixels! However, if we consider the additional field of view achieved by moving our eyes around a scene, thereby increasing the effective angle of view to as much as 120 degrees, a similar calculation produces an an answer in the region of 423 megapixels. (Note that such estimates are intended only as a broad guide, and should not be regarded as precise measurements.)

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