Levels Adjustment layer

How does a levels adjustment layer work?

Corrections with levels works best when the image is underexposed or overexposed

Corrections with levels is best done in each color channel to control changes to color saturation

Corrections with levels are limited to 3 points -- highlights, midtones, and shadows. Correction with curves is more complex but gives you more control.

Adjustment Layer Step for each color adjustment:

• Create a new adjustment layer by clicking Layer>New Adjustment Layer or by clicking the adjustment layer icon at the bottom of the layers palette (dark/light circle)
• Select Levels on the popup menu.

adjust color balance by setting the pixel distribution (histogram) for individual color channels.

• When you open the Levels dialog box you can see a histogram for the combined RGB image and separate histograms for red, green, and blue. Each shows the tonal range of the pixels in the image
• You can adjust the levels of each color channel individually or of the 3 channels (RGB) together. slider on the left end of each histogram indicates the tone that is black
• The little grey midtone slider (gamma) lets you leave highlight and shadow points as they are and remap the tonal arease between the 1/4 tone and 3/4 tone.
• Sometimes adjusting the gamma control greatly improves a badly exposed image
• Moving the gamma slider to the left makes the image lighter, by squeezing the highlights together and stretching out the tonal range of the shadows
• Be careful -- Dragging the gamma slider too far can result in posterization or banding by creating gaps in the histogram
• Try adjusting just the gamma slider
• Click OK

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Select the Levels layer

• Clipping Preview: Hold down the ALT key on the keyboard while sliding the sliders to see what colors are clipped or converted to black or white.
• The little white highlight slider at the right end of each histogram indicates the tone that is white
  • Hold down the alt key while you drag the slider to see the pixels that will be clipped (dropped) at each point. When you clip you lose color information.

• If you want to increase the contrast in the image, you can move the shadow and highlight sliders toward the center of the histogram
• That resets the black point and the white point and reconfigures the tonal range in between
• The Output Levels control decrease contrast.
• To change the whites in an image to black and the blacks to white, drag the black triangle to the right and the white triangle to the left.
• The image below shows the Histogram for the combined RGB channel.
• The images below that show the histograms for each channel separately.

Red channel levels

Green Channel levels

Blue Channel Levels

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Adjust the shadows and highlights manually

• To : You can set the highlights and shadows in an image by moving Input sliders to the first group of pixels on both ends of the Levels histogram.
• To make midtone corrections with the gamma slider (in the middle of the bar), use the gray Input Levels slider.
• Drag the slider to the right to darken the midtones
• drag it to the left to lighten the midtones
• Drag the black and white Input Levels sliders to the edge of the first group of pixels on either end of the histogram.

results in the image and RGB histogram after moving

• the Red midtone slider to the left to 1.31
• the green highlight slider to the left to the first pixels in the histogram at 218
• the blue midtone slider to the right to .70 to increase the green in the leaves
• You can now see gaps in the histogram where image data is lost -- if the gaps get too wide, the image may become posturized

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