Working with Levels


The Levels command displays an image histogram, which you can use as a visual guide to adjust the image s tonal range. Levels initially gives you three points of adjustment. The black slider on the left of the graph determines the darkest pixel in the shadow areas, which is called the black point. The white slider on the right determines the lightest pixel in the highlight area, called the white point. Move the black and white sliders to adjust the shadow and highlight extremes, respectively, of the image. The middle, or gamma, slider determines the median value between the black and white points. Move the slider to the right to decrease the median value, thereby making all values lower than the median darker , or to the left to increase it, making all values higher than the median lighter.

Note  

Before using the Levels command, choose Photoshop Preferences Memory & Image Cache. Be sure that the Use Cache For Histograms check box is cleared. Quit the program and then relaunch it. (See Chapter 5 for more on setting memory preferences.)

Whereas Input Levels increase contrast, Output Levels decrease contrast. Move the white slider to the left and the black slider to the right to reduce the range of contrast in an image. You can eliminate the extremes of the highlight and shadow in an image. Printers frequently do this to control ink coverage in preparing files for the press. For example, if the black arrow is moved from 0 to 12, values below 5% (equivalent to a 95% dot value) won t print.

When you perform a Levels adjustment, you are actually reassigning pixel values. As an example, suppose you have a low-contrast image such as the photograph in Figure 16.15.

click to expand
Figure 16.15: This picture definitely needs more contrast.
Note  

To best assess the results of a Levels adjustment, open the Histogram palette (Window Histogram) prior to applying the Levels command so that you can compare the before and after histograms.

Follow these basic steps to increase the contrast in this picture:

  1. Open the image in the h04 folder on the Photoshop CS Savvy CD titled duckybar.psd .

  2. Choose Window Histogram to display the Histogram palette. From the Palette Options menu, choose Expanded View. Click the Refresh arrows to display the histogram of the uncached data.

  3. Choose Image Adjustments Levels. The Levels dialog box displays an identical histogram. There is a deficiency in the highlight and shadow areas, where the absence of lines indicates there are no pixels of these brightness levels (see Figure 16.16.)

    click to expand
    Figure 16.16: The image, the Levels dialog box, and the Histogram palette, before the adjustment

  4. Move the white slider toward the center until it is aligned with the lines on the right of the graph, or until the Input Level box on the right reads about 207.

  5. Move the black slider toward the center until it is aligned with the lines on the left of the graph, or until the Input Level value on the left reads about 60.

The context-sensitive Histogram palette displays the new histogram superimposed over the previous one. The range of pixel values in the histogram has been redistributed to encompass the length of the entire graph. The lines that had a value of 60 now have a value of 0 (black), and the lines that had a value of 207 now have a value of 255 (white).

Figure 16.17 shows the Levels dialog box, the Histogram palette and the image after the correction. Compare it to Figure 16.16.

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Figure 16.17: The image, the Levels dialog box, and the Histogram palette after the adjustment

Adjusting Channels

If you perform a Levels adjustment on the composite channel, you have only three points of adjustment. If you adjust the levels of each channel individually, you have nine points of adjustment in an RGB or Lab color image and twelve points of adjustment on a CMYK image. This triples or quadruples the power of the Levels command; it can also produce weird color combinations. To choose a specific channel in which to work, scroll down the Channel list in the Levels dialog box. (See Hands On 5, Image Size, Transformation, and Color Adjustment, to try this operation.)

Note  

When adjusting individual channels, display the Histogram palette. Choose All Channels View from the Histogram palette menu to individually preview the result of each channel s histogram. Show Channels In Color will display the histogram of each channel in its corresponding color.

When adjusting the levels of individual channels, you might need to reset the Levels adjustment several times before producing the right combination of values. To do so, press the Option/Alt key. The Cancel button becomes the Reset button. Click it to begin again. Note that this cancels all the operations that you have performed in the dialog box, not just the individual channels. To cancel only the last operation, press /Ctrl-Z.

Determining the White Point and Black Point

You can use the Levels command in Threshold mode to locate the highlight and shadow areas of an image. You can then assign specific values to those points to redistribute all the other pixel values between those values.

Finding the Highlight and Shadow Points

Use this technique to determine the lightest and darkest areas of the image:

  1. Be sure that the composite RGB channel is selected from the Channels palette.

  2. Open the Levels dialog box (Image Adjustments Levels) and check the Preview box.

  3. Press Option/Alt and slowly drag the white Input Level slider to the left. A high-contrast preview appears. The visible areas of the image are the lightest part of the image.

  4. Repeat the process with the black shadow slider, dragging it to the right to identify the darkest areas of the image.

You can assign specific values to the darkest shadow areas and the lightest highlight areas of an image and then redistribute the brightness information based on the light and dark extremes of the image. Prepress professionals frequently determine CMYK values for highlight and shadow areas based on the characteristics of their printing presses. When you determine the white point, it is often best to use the lightest printable area of the image that contains detail, not a specular white that when printed will contain no ink. The shadow area will be the darkest area that contains detail, not an absolute black.

Setting the White Point

You can determine a specific RGB value for the white point of the lightest highlight areas:

  1. To set the target RGB values, open the image learjet.psd from the h04 folder on the CD. Before (Figure C16) and after (Figure C17) versions of this image are included in the color section.

  2. In the Eyedropper Options bar, set the Eyedropper tool to 3 By 3 Average.

  3. Choose Image Adjustments Levels.

  4. Double-click the white eyedropper. The Color Picker appears.

  5. Enter values for the highlight to prepare the image for print. Enter these recommended RGB values if you are printing on white paper: 240R, 240G, 240B. The grayscale density is a 6% dot (you can determine the highlight density by subtracting the Brightness value, B, in the Color Picker from 100).

    Note  

    The RGB values will vary depending on the RGB color space you are working in. I m working in Adobe RGB (1998) as a color setting (see Chapter 15). The point is, you re going to shoot for about a 4% to 6% neutral gray mix.

  6. Locate the lightest area on the image. Do not choose a specular highlight, which will be pure white; instead, choose the lightest area that will contain detail. In this case, click on the lightest area close to the nose cone of the jet to set the highlight.

Setting the Black Point

Determine a specific RGB value for the black point of darker shadows:

  1. Double-click the black eyedropper. The Color Picker appears.

  2. Enter RGB values for the shadow. Use these recommended values if you are printing on white paper and working in the Adobe RGB (1998) color space: 13R, 13G, 13B. The Grayscale density is a 95% dot. (As with the highlight value, you can determine the shadow density by the subtracting the Brightness value, B, in the Color Picker from 100.)

    Note  

    If the image is to be converted to CMYK, the total ink coverage should not exceed 300% in the CMYK percentage fields for a common web-coated separation. You can input other values of ink in the CMYK fields ”depending on the paper, printer, and press you are using ”but it is important to maintain three equal RGB values to assure neutrality before converting to CMYK.

  3. Locate the darkest area on the image that still contains detail. In this case, it s the dark circle of the left jet engine. Click your mouse there to set the shadow. Compare the results in Figure 16.18 (and in Figures C16 and C17 in the color section).

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Figure 16.18: The jet before (left) and after (right) determining the white and black points with the eyedroppers in the Levels dialog box.

The process for determining the white point and the black point is the same for both the Levels and the Curves operations. I ll discuss curves in the next section.

Saving and Loading Levels Settings

After you ve made a correction to the image, you might want to apply it to another image with the same color problems. Let s say you shot a roll of film at the wrong ASA and consequently underexposed all the images. You can adjust one image and apply those settings to the entire group by first saving and then loading the settings.

To save and load a setting:

  1. Choose Save from the options on the right side of the Levels dialog box.

  2. Choose a folder in which to save the settings, name them, and click OK. Click OK again to close the Levels dialog box. Now that you ve saved the settings, you can reload them at any time.

  3. To load the settings, choose Load from the options on the right side of the Levels dialog.

  4. Locate the folder where the settings were saved, and click Open to open the settings.




Photoshop CS Savvy
Photoshop CS Savvy
ISBN: 078214280X
EAN: 2147483647
Year: 2004
Pages: 355

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