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Getting the Best Grayscales in Photoshop CS3

  • Sample Chapter is provided courtesy of Adobe Press.
  • Date: Feb 1, 2008.

Chapter Description

Trusted instructor Ben Willmore shares his techniques for optimizing grayscale images in Photoshop CS3.

Setting Up Your Images for Final Output

If your images are going to be printed on a commercial printing press, chances are that they will end up looking a lot darker than they did when you viewed them onscreen. This is known as dot gain. Fortunately, Photoshop allows you to compensate for it. You can tell Photoshop ahead of time how you intend to output your images, and it will adjust the onscreen appearance of your image to look as dark as it should be after it's printed.

To select or enter dot gain settings, choose Edit > Color Settings. In the Working Spaces area, you'll use the Gray pop-up menu (Figure 4.21). You'll definitely want to ask your printing company about what settings to use; otherwise, you'll just be guessing and you might not like your end result. But just in case you don't have time to ask your printing company, you can use the settings that appear in Table 4.1. After you've specified the Dot Gain setting that is appropriate for your printing conditions, choose Image > Mode > Assign Profile, and select the Working Gray setting. That will set up Photoshop to properly preview what your image will look like under those conditions.

Figure 4.21

Figure 4.21 The Color Settings dialog box.

Table 4.1. Dot Gain Settings

Newspapers

34%

Magazines and brochures

24%

High-end brochures

22%

Preparing for a Printing Press

Take a close look at the black-and-white image in Figure 4.22, and imagine that you took that image to Kinko's and made a copy of it. Then you took the copy and copied it again at your local library. Then you took the library copy and ran it through the copy machine in your office. Then you held the version that had been copied three times next to the original. Would you expect them to look the same? Of course not. In fact, the tiny dots that are in the brightest part of the image would have begun to disappear and become pure white, because every time you make a copy, you lose some quality. Well, the same thing happens when you hand over your image to a printing company. When you give your printing company your original output, it has to make three copies of it before it makes it to the end of the printing process. The company starts by converting the original into a piece of metal called a printing plate to make the first copy. Then the plate is put on a big, round roller on the printing press and flooded with water and ink. The oily ink sticks to the plate only where your images and text should be; the water makes sure it doesn't stick to the other areas (using the idea that oil and water don't mix). Next to that roller is another one known as a blanket; it's just covered with rubber. The plate comes into contact with the blanket so the ink on the plate will transfer over to the blanket—that's your second copy. Finally, the blanket transfers the ink onto a sheet of paper to create the last copy (Figure 4.23). Each time a copy is made, you lose some of the smallest dots in the image. Until you know how to compensate for this, you're likely to end up with pictures of people with big white spots in the middle of their foreheads.

Figure 4.22

Figure 4.22 Copy this image three times and you'll lose detail in the brightest part of the image. (©2007 Ben Willmore)

Figure 4.23

Figure 4.23 Three copies are made before your image turns into a printed page.

Before I show you how to compensate for the loss of detail in the bright areas of your image, let's look at what happens to the darkest areas, since we'll have to deal with them as well. When you print with ink on paper, the ink always gets absorbed into the paper and spreads out—just like when you spill coffee on your morning newspaper. This causes the darkest areas of an image (97%, 98%, 99%) to become pure black. If you don't adjust for this, you will lose detail in the shadows of your image.

Most printing companies create a simple test strip that it prints on the edge of your job in the area that will be cropped after it's printed. This test strip contains shades of gray from 1% to about 5% to determine the lightest shade of gray that doesn't disappear on press and become pure white. Of course, the folks in the printing industry don't just use plain English to describe it; instead, they invented the term "minimum highlight dot reproducible on press." The test strip area also contains shades of gray from 99% to about 75% so they can see the darkest shade of gray that doesn't become pure black. For that one, they came up with the term "maximum shadow dot reproducible on press." If you ask your printing company, it can usually tell you exactly which settings to use. I know you don't always know who will print your images or don't have the time to ask, so I'll give you some generic numbers to use (Tables 4.2 and 4.3). But first, let's find out how we adjust for minimum highlight and maximum shadow dots.

Table 4.2. Common Minimum Highlight Settings

Newspapers

5%

Magazines and brochures

3%

High-end brochures

3%

Table 4.3. Common Maximum Shadow Settings

Newspapers

75%

Magazines and brochures

90%

High-end brochures

95%

By moving the lower-right slider in the Levels dialog box, you will change white to the shade of gray the slider is pointing to. You want to move this slider until it points to the minimum highlight dot—that is, the lightest shade of gray that will not disappear and become white on-press.

You don't want to eyeball this setting, so instead of just looking at the shades of gray, we'll use the Output Level numbers in the Levels dialog box. There is one problem with these numbers: They range from 0 to 255 instead of 0 to 100%! This is because you can have up to 256 shades of gray in a grayscale image, and Photoshop wants you to be able to control them all. When you're using this numbering system, think about light instead of ink. If you have no light (0), it would be pitch black; if you have as much light as possible (255), you could call that white. So that you won't need a calculator, I'll give you a conversion table (Table 4.4).

Table 4.4. Percentage Conversion Table

100%

0

99%

3

98%

5

97%

8

96%

10

95%

13

94%

15

93%

18

92%

20

91%

23

90%

26

89%

28

88%

31

87%

33

86%

36

85%

38

84%

41

83%

44

82%

46

81%

49

80%

51

79%

54

78%

56

77%

59

76%

61

75%

64

74%

67

73%

69

72%

72

71%

74

70%

77

69%

79

68%

82

67%

84

66%

87

65%

90

64%

92

63%

95

62%

97

61%

100

60%

102

59%

105

58%

108

57%

110

56%

113

55%

115

54%

118

53%

120

52%

123

51%

125

50%

128

49%

131

48%

133

47%

136

46%

138

45%

141

44%

143

43%

146

42%

148

41%

151

40%

154

39%

156

38%

159

37%

161

36%

164

35%

166

34%

169

33%

172

32%

174

31%

177

30%

179

29%

182

28%

184

27%

187

26%

189

25%

192

24%

195

23%

197

22%

200

21%

202

20%

205

19%

207

18%

210

17%

212

16%

215

15%

218

14%

220

13%

223

12%

225

11%

228

10%

230

9%

233

8%

236

7%

238

6%

241

5%

243

4%

246

3%

248

2%

251

1%

253

0%

255

By moving the lower-left slider in the Levels dialog box, you will change black to the shade of gray the slider is pointing to (Figure 4.24). You want to move this slider until it points to the darkest shade of gray that will not plug up and become black (known as the maximum shadow dot).

Figure 4.24

Figure 4.24 The bottom sliders reduce image contrast to compensate for the limitations of the printing press.

At first glance this stuff might seem complicated, but it is really quite simple. All you do is use the numbers from the tables or ask your printing company for settings. If you always print on the same kind of paper, you'll always use the same numbers.

5. A Quick Levels Recap | Next Section Previous Section

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