You use the Levels adjustment 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. For more information on how to read a histogram, see About histograms.
A. Shadows B. Midtones C. Highlights D. Output Level sliders
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 black, and level 255, where the pixels are white. With the Output sliders in the default positions, 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.
When shadows are clipped, the pixels are black, with no detail. When highlights are clipped, the pixels are white, with no detail.
The middle Input slider adjusts the gamma in the image. It moves the midtone (level 128) and changes the intensity values of the middle range of gray tones without dramatically altering the highlights and shadows.
Choosing Image > Adjustments > Levels makes direct adjustments to the image layer and discards image information.
For example, if you move the black point slider to the right at level 5, Photoshop maps all the pixels at level 5 and lower to level 0. Similarly, if you move the white point slider to the left at level 243, Photoshop maps all pixels at level 243 and higher to level 255. The mapping affects the darkest and lightest pixels in each channel. The corresponding pixels in the other channels are adjusted proportionately to avoid altering the color balance.
You can also enter values directly into the first and third Input Levels text boxes.
Moving the middle Input slider to the left makes the overall image lighter. This slider adjustment maps a lower (darker) level up to the midpoint level between the Output sliders. If the Output sliders are in their default position (0 and 255), the midpoint is level 128. In this example, the shadows expand to fill the tonal range from 0 to 128, and the highlights are compressed. Moving the middle Input slider to the right has the opposite effect, making the image darker.
You can also enter a gamma adjustment value directly in the middle Input Levels box.
You can view the adjusted histogram in the Histogram panel.
In the Adjustments panel, click the Levels icon or choose Levels from the panel menu.
In the Properties panel, do one of the following to neutralize a color cast:
In general, assign equal color component values to achieve a neutral gray. For example, assign equal red, green, and blue values to produce a neutral gray in an RGB image.
If the image needs overall contrast because it doesn’t use the full tonal range, click the Levels icon in the Adjustments panel. Then drag the Shadow and Highlight input sliders inward until they touch the ends of the histogram.
A. Shadow Input slider B. Highlight Input slider