Photographs Vs Human Vision

Photographs are often considered to be accurate portrayals of visual scenes. However, photographs do not record images in the same way the human eye would respond to the identical scenes. For the following discussion, we ignore special techniques, such as filters, infrared, etc. The discussion focuses on natural-light photography under sunlit conditions. There are several fundamental ways in which photographic images differ from what is seen by the human eye.

• Field of view: Camera lens focal length determines the field of view which is focused onto the film or electronic detector array. A focal length of ~ 50 mm (for 35 mm format film) approximates the central zone of the human visual field. A wide-angle lens compresses more field of view onto the image plane; whereas a telephoto lens severely limits (crops) the field of view. The human eye provides for an extremely wide angle of peripheral vision, up to ~ 160° in most people. This peripheral vision is good for recognition of gray-tone patterns and is highly sensitive for detection of movement, but has almost no color capability. Photographs do not provide a similar peripheral field of view—the softly edged elliptical viewing field of the photographer is sharply trimmed to the rectangular extent of the image frame.

• Latitude: This refers to the range of highlights and dark features that are properly exposed in a photograph. Color-slide film has a total latitude of about five f-stops (Shaw, 1994). Assuming the mid-tone of the scene is correctly exposed, this means that brighter or darker features also appear correct within ±2.5 stops. Features more than 2.5 stops brighter would be "burned out'' (pure white) and objects more than 2.5 stops darker are all black. Digital cameras generally have more restricted latitude and a greater tendency to burn out for bright objects compared with film. The human eye, in contrast, has much greater latitude equivalent to 1214 f-stops.

• Color saturation: Our visual system adapts itself to changes in light intensity and color temperature, but the color rendition of color film may be different from the human eye. People tend to prefer vibrant, rich, saturated colors in photographs. Filmmakers have responded to this preference in different ways. Some manufacturers favor color-saturated films, whereas, other films render color as close as possible to reality (Zuckerman, 1996). Similar differences are apparent in color recorded by digital cameras (see Chapter 6).

Given these and other factors, it should be clear that a photographic image of a scene is different in several ways from the human visual impression of that same scene. The photograph represents a selection of certain elements—field of view, exposure range, and color saturation, which are in general more limited than a human observer would sense. On the other hand, a photograph is a permanent record of the scene, while the human visual impression is stored as memory that cannot be reproduced fully or accurately for analysis or sharing with others.

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