Dynamic range and human perception
The human senses of
sight and hearing have a very high dynamic range. A person is capable of hearing (and usefully discerning) anything from a quiet murmur in a soundproofed room to the sound of the loudest rock concert. A difference like this can be 100
dB which represents a difference in energy of 10,000,000,000.
Equally a person can see objects in starlight (although colour differentiation is reduced at low light levels) or in bright sunlight, even though on a moonless night objects receive 1/1,000,000,000 of the illumination they would on a bright sunny day: that is a dynamic range of 90 dB. A person cannot perform these feats of perception at both extremes of the scale at the same time. The eyes take time to adjust to different light levels and the dynamic range of the human eye without any adjustment of the pupil is only approximately 30 dB. The instantaneous dynamic range of human audio perception is similar, so that, for example, a whisper cannot be heard in loud surroundings. Nevertheless, a good quality audio reproduction system should be able to reproduce accurately both the quiet sounds and the loud;
and a good quality visual display system should be able to show both shadow details in nighttime scenes and the full brightness of sunny scenes.
In practice it is difficult to achieve the full dynamic range seen by human beings using electronic equipment, since most electronic reproduction equipment is essentially linear rather than logarithmic like human perception. Electronically reproduced audio and video often uses some trickery to fit original material with a wide dynamic range into a narrower recorded dynamic range that can more easily be reproduced: this is dynamic compression. For example a good quality LCD display has a dynamic range of around 1000, or 30 dB
(commercially the dynamic range is often called the "contrast ratio" meaning the full on/full off contrast ratio). When showing a movie or a game such a display is able to show both shadowy nighttime scenes and bright outdoor sunlit scenes, but in fact the level of light coming from the display is much the same for both types of scene (perhaps different by a factor of 10). Knowing that the display does not have a huge dynamic range, the program makers do not attempt to make the nighttime scenes millions of times less bright than the daytime scenes, but instead use other cues to suggest night or day: a nighttime scene will contain duller colours and will often be lit with blue lighting, which reflects the way that the human eye sees colours at low light levels.
Photography
Main article: Exposure range
Photographers use
exposure range as a
synonym for the
luminosity range of a scene being photographed; the
light sensitivity range of photographic film, paper and digital camera sensors; the
opacity range of developed film images; the
reflectance range of images on photographic papers.
In
photography,
exposure range is one of several types of
dynamic range:
- The Light sensitivity range of photographic film, paper, or digital camera sensors.
- The luminosity range of a scene being photographed.
- The opacity range of developed film images
- The reflectance range of images on photographic papers.
A
graduated neutral density filter can be used to control exposure range.
The exposure range of a device is usually expressed in stops, which are equivalent to log2(
c) where
c is the medium or device's
contrast ratio. For example, average
Digital Video (DV) has a contrast ratio of 45:1, so its exposure range is roughly 5.5 stops.
Film has an exposure range of 11 stops, currently the highest in motion picture mediums.
Shown are three "exposures" of the same HDR image: -4
f-stops, the desired exposure, and +3 f-stops.
http://en.wikipedia.org/wiki/Dynamic_range
http://en.wikipedia.org/wiki/Exposure_range
και μεταφρασμενο απο το google
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