1 Open the Levels dialog box, and select Preview.
Note: The Threshold mode in Levels is not available for CMYK images.
2 Hold down Alt (Windows) or Option (Mac OS), and drag the white or black Input Levels triangle.
The image changes to Threshold mode, and a high-contrast preview image appears. The visible areas of the image indicate the lightest parts of the image if you are dragging the white slider, and the darkest parts if you are dragging the black slider. If a color channel is selected in the Levels dialog box, the black area indicates where none of the given color component exists.
3 Slowly drag the slider to the center of the histogram to identify the light or dark areas in the image. Use these pixels for targeting the black point and white point in your image.
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