Levels
The Levels dialog box lets you correct the tonal range and colour balance of an image by adjusting intensity levels of image shadows, midtones, and highlights. Select Image menu > Adjust sub menu > Levels command. In the dialog box that opens up, you will see the levels histogram displaying the distribution of colour pixels in a two dimensional graph. To get proper correction in Photoshop, you want to understand your image’s “black point” and “white point.” The black point is the darkest portion of your image and the white point is the brightest highlight of your image. The information between the black point and the white point is known as the dynamic (or tonal) range of your photograph.
In some cases, you can use a quick fix to an image by clicking on the Auto button at the right hand side. The Auto Levels command automatically adjusts the black point and white point in an image. This clips a portion of the shadows and highlights in each channel and maps the lightest and darkest pixels in each colour channel to pure white (level 255) and pure black (level 0). The intermediate pixel values are redistributed proportionately. As a result, using Auto Levels increases the contrast in an image because the pixel values are expanded. Auto Levels gives good results in certain images with an average distribution of pixel values that need a simple increase in contrast. You can always go into each channel and manually reposition the black, white and mid-grey input sliders.
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You can add contrast to an image in two ways, depending on the problem:
- If the image needs overall contrast because it doesn’t use the full tonal range (the graph doesn’t extend to the ends), choose Image menu> Adjustments submenu > Levels. Then drag the Shadow and Highlight input sliders inward until they touch the ends of the histogram.
- Another way to add contract to the image is using the Curves dialog box. The Curves dialog box is better used to increase contrast in the midtones by increasing the slope in the middle of the curve.