Assigning (targeting) highlight and shadow values of an image is necessary because most output devices (usually printing presses) cannot print detail in the blackest shadow values (near level 0) or the whitest highlight values (near level 255). Specifying the minimum shadow level and maximum highlight level helps to bring the important shadow and highlight details within the gamut of the output device.
If you are printing an image on a desktop printer and your system is color-managed, don’t set target values. The Photoshop color management system automatically makes adjustments to the image you see on the screen so that it prints properly on your profiled desktop printer.
The Output Levels sliders let you set the shadow and highlight levels to compress the image into a range less than 0 to 255. Use this adjustment to preserve the shadow and highlight details when an image is being printed on a press whose characteristics you know. For example, suppose there are important image details in the highlights with a value of 245, and the printing press that you’re using won’t hold a dot smaller than 5%. You can pull the highlight slider to level 242 (which is a 5% dot on the press) to shift the highlight detail from 245 to 242. Now, the highlight detail can safely print on that press.
Generally, it is not a good idea to use the Output Levels sliders to target images with specular highlights. Your specular highlight will look gray rather than blow out to pure white. Use the highlight eyedropper for images with specular highlights.
in
the toolbox. You can choose 3 by 3 Average from the Sample Size
menu in the Eyedropper tool options. This ensures a representative
sample of an area rather than the value of a single screen pixel.
or
Curves icon 
in
the Adjustments panel.
When you select Levels or Curves, the Eyedropper tool is active outside the Properties panel. You still have access to the scroll controls, the Hand tool, and the Zoom tool 
through keyboard shortcuts.
Move the pointer around the image, and look at the Info panel to find the lightest and darkest areas that you want preserved (not clipped to pure black or white). (See View color values in an image.)
Drag the pointer in the image, and look at Curves in the Properties panel to find the lightest and darkest points you want to preserve. This method does not work if the Curves adjustment is set to the CMYK composite channel.
When identifying the lightest highlight details that you want targeted to a printable (lower) value, don’t include specular highlights. Specular highlights such as the highlight glint in jewelry or a spot of glare are meant to be the brightest points in an image. It’s desirable to clip specular highlight pixels (pure white, no detail) so that no ink is printed on the paper.
You can also use the Threshold command to identify representative highlights and shadows before accessing Levels or Curves. (See Create a two-valued black and white image.)
in
the Levels or Curves adjustment to display the Color Picker. Enter
the values you want to assign to the lightest area in the image,
and click OK. Then click the highlight you identified in
step 3.
If you accidentally click the wrong highlight,
click the Reset button 
in
the Adjustments panel.
Depending on the output device, you can achieve a good highlight in an average-key image using CMYK values of 5, 3, 3, and 0, respectively, when you are printing on white paper. An approximate RGB equivalent is 244, 244, 244, and an approximate grayscale equivalent is a 4% dot. You can approximate these target values quickly by entering 96 in the Brightness (B) box under the HSB area of the Color Picker.
With a low‑key image, you may want to set the highlight to a lower value to avoid too much contrast. Experiment with Brightness values from 96 through 80.
The pixel values are adjusted throughout the image proportionately to the new highlight values. Any pixels lighter than the area you clicked are clipped (adjusted to level 255, pure white). The Info panel shows the values both before and after the color adjustment.
To assign shadow values to the darkest area of the image that you want preserved, double-click the Set Black Point Eyedropper tool 
in the Properties panel to display the Color Picker. Enter the values you want to assign to the darkest area in the image, and click OK. Then click the shadow you identified in step 3.
When you’re printing on white paper, you can usually achieve a good shadow in an average-key image using CMYK values of 65, 53, 51, and 95. An approximate RGB equivalent is 10, 10, 10, and an approximate grayscale equivalent is a 96% dot. You can approximate these values quickly by entering 4 in the Brightness (B) box under the HSB area of the Color Picker.
With a high‑key image, you may want to set the shadow to a higher value to maintain detail in the highlights. Experiment with Brightness values from 4 through 20.
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