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Old 19-12-2016, 11:22 AM
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RickS (Rick)
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Join Date: Apr 2010
Location: Brisbane
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The data from a CCD or CMOS sensor is "linear." That means that the value in each pixel is linearly proportional to the number of photons detected. If one pixel detects ten times as many photons as another one, then it will have a value (in ADU) that's ten times larger.

Our eyes don't respond to light linearly like a camera sensor. Their response is often described as logarithmic but the reality is much more complex than this (see http://www.telescope-optics.net/eye_...y_response.htm for gory details.) Stretching is the process of converting linear sensor data to something that matches our vision, boosting low signal values disproportionally.

The methods available to perform this stretching vary between processing packages. In PS it is typically done, as you said, with levels and curves.

Cheers,
Rick.
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