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The human eye is a remarkable organ, and proves itself to be incredibly flexible as far as dynamic range is concerned. It enables us to perceive a ten-stop range of light intensity (1,000:1) under almost any circumstances, and a much greater range when numerous other factors are taken in to account. The dynamic range depends to a large extent upon contrast, and under low light conditions it may not be much in excess of 1,000 : 1. In bright daylight giving good contrast, the range may rise to as much as 10,000 : 1.

It is important to remember that, in a manner comparable with a camera, the eye also adapts itself to light intensity. The iris closes much like a lens diaphragm in bright light, and opens when the intensity of illumination decreases. Although this increases the capability of the eye to handle a much larger dynamic range, the brighter and darker extremes of the range are not detectable simultaneously. Our common experience is that we can see very bright objects on a sunny day, and also detect thousands of stars and galaxies in the night sky. This represents a dynamic range of range arguably exceeding 10 million : 1. However, we also know that when we leave a brightly-lit room and go outside to view the night sky, it is necessary to give our eyes time to adapt to the much lower light levels. Similar flexibility is available with a camera given extensive changes of shutter speed, aperture and ISO rating.

In practice, the human eye can perform staggering feats of perception. We can see objects on moonless night when the illumination is provided by the stars of the Milky Way. However, under such low light conditions colour perception falls away and may virtually disappear. We can also see clearly subjects such as white sand beaches lit by direct sunlight, where the level of illumination may be hundreds of millions of times higher. The conclusion, therefore, is that the dynamic range of the human eye is far greater than that achievable by any film or digital camera.

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