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TOPIC: Lighting and Colour Adjustment 101 part III

Lighting and Colour Adjustment 101 part III 1 year 4 months ago #1265

  • Ian Wilson
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In the previous posts on this subject I showed two simple ways to keep an eye on the colour channels, in particular to see if any channels were saturated in part of the image. Photoshop provides another option that enables one to visualize those parts of an image that might be saturated. This uses the threshold screen in Levels. To active this threshold screen one must first go to Enhance/Adjust Lighting/Levels to open the Levels adjustment window. See attached image. The Levels adjustment window shows the RGB histogram of the image. Make sure the Preview box is ticked and then hold down the Alt key on your keyboard and left-click your mouse on the 255 DN arrow head on the horizontal axis of the histogram. This will turn on a black threshold screen which only allows the display of image pixels having a brightness of 255 DN. These are the pixels in the image that have reached saturation. If there is a significant part of the image that is saturated then that part of the image will be displayed against the black threshold screen. Furthermore, the saturated parts of the image appear coloured, for example, those pixels on the robin's breast that are saturated appear red against the black background indicating that some pixels in the red channel are saturated. Similarly, if any green or blue pixels were saturated they would be displayed in green or blue against the black background. If all three colour channels are saturated at a particular point in the image then these pixels will appear as a white patch against the black background.

This is a really neat way of quickly checking the entire image for pixels that might be saturated. Similar threshold screens can be found in ACR and Light Room where they are used for setting the black and white point for the image.
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