Adjust > Levels
You use the Levels dialog box to correct the tonal range and color balance of an image by adjusting intensity levels of image shadows, midtones, and highlights. The Levels histogram is a visual guide for adjusting the image key tones.
1. Do one of the following:
 Choose Adjust > Levels
 Click the Levels button on the toolbar
2. Use the following options:
Input / Output Levels
The outer two Input Levels sliders map the black point and white point to the settings of the Output sliders. By default, the Output sliders are at level 0, where the pixels are completely black, and level 255, where the pixels are completely white. So, in the default position for the Output sliders, moving the black input slider maps the pixel value to level 0, and moving the white point slider maps the pixel value to level 255. The remaining levels are redistributed between levels 0 and 255. This redistribution increases the tonal range of the image, in effect increasing the overall contrast of the image.
Use this option to edit the red, green, and blue channels. Choose Red, Green, or Blue to edit the individual color channels.
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