Easy Photoshop Tutorial: Optimizing grayscale images

optimizing grayscales in CS3 photoshop tutorial

  • If you go through life convinced that your way is always best, all the new ideas in the world will pass you by.
  • —Akio Morita, founder of Sony

When you work with grayscale images, your adjustments and corrections are limited to only tonal adjustments—that is, changes to the brightness and contrast of the image. When you work with color images, you also have to worry about hue, saturation, and the other properties of color.

Photoshop provides several tools for making tonal adjustments, and we’ll look at all of them. You can use these tools for correcting and adjusting both grayscale and color images.

Brightness/Contrast

With all previous versions of Photoshop, it was safe to say that you should never use the Brightness/Contrast dialog box to make tonal adjustments. This was because Brightness/Contrast used to adjust the entire tonal range of the image by equal amounts, which made it difficult to adjust one part of the image—say, the shadows—without screwing up another part of the image, such as the highlights.

With CS3, Adobe has reengineered the Brightness/Contrast dialog box and turned it into a very useful, very powerful tonal adjustment tool.

You’ll find the Brightness/Contrast adjustment by choosing Image > Adjustments > Brightness/Contrast. The dialog box is very straightforward (Figure 4.1). By sliding the Brightness slider back and forth, you can make your overall image brighter or darker. In general, the Brightness slider protects your shadow areas—it won’t usually let you underexpose them too far. This means that you need to keep a very close eye on the highlights in your image. As you adjust the slider, be careful that you don’t let the highlights overexpose and blow out to complete white (Figure 4.2).

optimizing grayscales in CS3 photoshop tutorial

Figure 4.1 The Brightness/Contrast dialog box.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.3 With the Contrast slider you can add more punch to your images.

Moving the slider to the left lowers the contrast, resulting in a flatter image (Figure 4.4).

optimizing grayscales in CS3 photoshop tutorial

Figure 4.4 The Contrast slider can also be used to pull contrast out of an image.

Brightness/Contrast is not the most refined tool, but it can be a great place to start if you’re new to Photoshop.

optimizing grayscales in CS3 photoshop tutorial

Levels

Brightness/Contrast is especially useful if you’ve never performed tonal corrections before, and on many images, it’s all the control you’ll ever need. However, Photoshop’s Levels adjustment provides a more sophisticated tool that offers a much finer degree of control (Figure 4.5).

optimizing grayscales in CS3 photoshop tutorial

Figure 4.5 Understanding the Levels sliders.

With Levels you have five different sliders that you can adjust, as well as a histogram (sort of like a bar chart) (Figure 4.6) that indicates exactly what is happening to the image.

The Histogram Is Your Guide

You can use the bar chart (also known as a histogram) at the top of the Levels dialog box to determine whether the adjustments you’re making are going to harm the image or improve it. The histogram indicates which shades of gray your image uses and how prevalent those shades are within the image. If you find a gap in the histogram, you can look at the gradient directly below it to see which shade of gray is missing from your image.

By looking below the left side of the histogram, you can determine the darkest shade of gray in the image. By looking below the right end of the histogram, you can determine the brightest shade of gray in the image. If you look at Figure 4.7, you might notice the image contains no pure blacks or pure whites. The darkest shade of gray is about 95%, and the brightest shade is about 6%.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.6 This histogram indicates that the shades between around 90% and 75% gray take up a lot of space (tall bars), and the shades between around 5% and 15% take up little space (short bars).

NOTES

To reset sliders to their default positions, hold down Option/Alt to temporarily change the Cancel button to a Reset button.

If you have a color print or transparency that will be reproduced as a grayscale image, be sure to scan the original as color and then convert it to grayscale in Photoshop. Also, be sure to check out Chapter 9, “Color Manipulation,” where you’ll learn how to produce a higher-quality grayscale conversion.

The height of the bars in a histogram suggest, visually, how much space the shades take up in an image. The height doesn’t indicate an exact number of pixels; instead, it measures how much that shade is used compared with the other shades in the image. It’s as if everyone in a room stood up and you compared how tall each person was without using a ruler. You wouldn’t know exactly how tall anyone was, but you’d have an idea of how tall each person was compared with the others.

Figure 4.7 Look at the gradient bar directly below the ends of the histogram to determine the brightest and darkest shades present in the image.

There is no ideal when it comes to a histogram; it’s simply a representation of which shades of gray are most prevalent in your image (Figure 4.8). The peaks indicate a shade of gray that takes up a lot of space in the image, and the valleys indicate a shade that isn’t very prevalent in the image. A histogram that extends all the way across the space available and does not have tall spikes on either end indicates an image that has the full range of shades available, and is usually a sign of a good scan or a well-adjusted image.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.8 Each image will have its own unique histogram.

Evaluating and Adjusting Contrast

The brightest and darkest areas of your computer monitor are nowhere near as bright or dark as the objects you’ll find in the real world. The difference is even more extreme when you look at the brightest and darkest areas of a printed brochure—the paper is actually pretty dull, and the ink isn’t all that dark. Because of this, you’ll need to use the full range of shades from black to white in order to make your photos look as close to reality as possible.

NOTES

The middle slider will move when you adjust the upper-right or upper-left slider. This happens because Photoshop is attempting to keep the middle slider in the same position relative to the other two sliders. So if the middle slider is centered between the other two, it will remain centered when you move one of the outer sliders.

By adjusting the upper-right and upper-left sliders in the Levels dialog box, you can dramatically improve the contrast of an image and make it appear more lifelike. When you move the upper-left slider in the Levels dialog box, you force the shade of gray directly below it and any shade darker than it (see the gradient) to black. So moving that slider until it touches the first bar on the histogram forces the darkest shade of gray in the image to black, which should give you nice dark shadows.

When you move the upper-right slider, you force the shade that appears directly below the slider and any shade brighter than it to white. So, similar to dark colors, moving the right slider until it touches the last bar on the histogram forces the brightest shade of gray to white, which should give you nice white highlights.

By adjusting both sliders, your image will use the full range of shades available to a grayscale image (Figure 4.9). If you move the sliders past the beginning and end of the histogram, you will get even more contrast, but you risk losing important detail in the process.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.9 The shades that are beyond the upper-right and upper-left triangles become pure black and pure white.

Hidden Features to the Rescue

To achieve maximum contrast without sacrificing detail, Adobe created a hidden feature in the Levels dialog box. It’s known as Threshold mode. This feature allows you to see exactly which areas are becoming black or white, and it’s the key to ensuring that you don’t sacrifice detail. To get to the hidden feature, hold down the Option/Alt key when you move the upper-right or upper-left sliders in the Levels dialog box.

NOTES

If you’re in the market for a new scanner, be sure to compare the D-max specifications for each scanner you are considering. Higher D-max specs indicate a scanner that is capable of capturing more shadow detail than a scanner with a lower D-max spec. If you can’t find the D-max specification on the manufacturer’s Web site, there’s a good chance that it’s too low to be proud of (for the same reason you don’t find 0–60 ratings in brochures for economy cars). It’s often worth the extra money to get a scanner that can deliver good shadow detail.

When you move the upper-left slider with Threshold mode turned on, your image should turn white until the slider touches the first bar on the histogram; then small black areas should start to appear. These are the areas that will become pure black. With most images, you’ll want to make sure you don’t force a large concentrated area to black, so move the slider until only small areas appear. You also want to make sure the areas that are becoming black still contain detail. Detail will show up looking like noise (not the kind you hear—the kind you see on an old television when you don’t have an antenna hooked up), so make sure those small areas also look noisy. You’ll need to repeat this process with the upper-right slider to make sure you get optimal contrast (Figures 4.10 to 4.14).

optimizing grayscales in CS3 photoshop tutorial

Figure 4.10 Original. (©2007 Ben Willmore)

optimizing grayscales in CS3 photoshop tutorial

Figure 4.11 Upper-left slider adjusted way too far.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.12 Large areas of the image are losing detail and becoming pure black.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.14 Small areas become black but still contain detail (noise).

Three things might cause an image to have large areas of black or white from the start:

  1. Your scanner isn’t capable of capturing good shadow detail.
  2. The image simply didn’t have any detail in the shadows to begin with.
  3. The image has been adjusted without using Photoshop’s Threshold mode.

The Histogram Gives You Feedback

After you have applied an adjustment to your image, you can see an updated histogram by choosing Image > Adjustments > Levels again. Notice that after adjusting the upper-right and upper-left sliders, the histogram stretches all the way across the area available. It’s just like stretching out a Slinky—you remember, “It walks downstairs, alone or in pairs” (Figure 4.15). As you pull on the ends of the Slinky, the loops stretch out and start to create gaps. The same thing happens to a histogram—because Photoshop can’t add more bars to the histogram, it can only spread out the ones that were already there. And remember, gaps in the histogram mean that certain shades of gray are missing from the image. So the more you adjust an image using Levels, the more you increase the possibility that you’ll lose some of the smooth transitions between bright and dark areas (Figure 4.16).

optimizing grayscales in CS3 photoshop tutorial

re 4.15 A Slinky.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.16 After adjusting the top two sliders, your image should use the full range of shades available.

If you see large spikes on either end of the histogram (Figure 4.17), it’s an indication that you’ve lost detail. That’s because you forced quite a bit of space to white or black using Levels. But you’d know you did that, because you used the hidden feature, right? Or maybe you couldn’t control yourself and used that Brightness/Contrast dialog box, where you can’t tell if you damaged the image! You might also get spikes on the ends of the histogram (Figure 4.18) if you scan an image with too high of a contrast setting or a brightness setting that is way too high or low, or if your scanner isn’t capable of capturing enough shadow detail.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.17 Spikes on the end of a histogram usually indicate lost detail.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.18 Noise.

NOTES

If you’d like to see a histogram that continuously changes to reflect any modifications you make to your image, then choose Window > Histogram.

If you find evenly spaced spikes in the histogram of an unadjusted image, it usually indicates a noisy scan (Figure 4.18).

Adjusting Brightness

After you have achieved good contrast, your image might look too bright or dark. The middle slider in the Levels dialog box can fix that. (Techies love to call this slider the Gamma setting, but we plain folks call it the midpoint.) If you move the middle slider to the left, the image becomes brighter without messing up the dark areas of your image. Black areas stay nice and black. Or you can move the middle slider to the right to darken the image without messing up the bright areas. White areas stay bright white (Figure 4.19). This is the one setting that is a personal choice. I can’t tell you how bright or dark your image should be.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.19 Effects of the middle slider.

If you want to know what this adjustment is doing, just look directly below the middle slider; the shade of gray there will become 50% gray. Moving it to the left brightens your image because you’ll be shifting what used to be a dark shade of gray to 50% gray. Moving the middle slider to the right darkens your image as you shift a bright shade to 50% gray. If you look at an updated histogram of the image, it will look like you stretched out a Slinky, then grabbed one side and pulled it to the middle (Figure 4.20). Some bars will get scrunched (is that a technical term?) together, whereas others get spread apart.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.20 The adjustment shown on the left results in the histogram shown on the right.

NOTES

Spikes that show up after an image has been adjusted with Levels do not indicate noise. It’s as if you took your trusty Slinky and tried to squish it down to a centimeter wide. Something would have to budge. The only way I can do it is to bend the Slinky into a V shape where the loops start piling up, one on top of the other. Otherwise, the loops just line up in a nice row and limit how much I can compress the Slinky. Well, the same thing happens with the histogram. Let’s say you try to squish 20 bars into a space that is only 15 pixels wide on the histogram. Five of the bars have to disappear. They are going to just pile on top of the bars next to them and make those bars about twice as tall. When this happens, you get evenly spaced spikes across part of the histogram.

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.

optimizing grayscales in CS3 photoshop tutorial

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.

optimizing grayscales in CS3 photoshop tutorial

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

optimizing grayscales in CS3 photoshop tutorial

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

NOTES

If the dot gain setting you need isn’t listed in the Working Spaces area, you’ll need a custom setting. Turn on the Advanced Mode check box at the top of the dialog box, and then choose Custom Dot Gain from the Gray pop-up menu. To get a traditional dot gain measurement (in which you measure only 50% gray), just add 50 to the dot gain setting you need, and enter the result in the 50% field.

If your image will be displayed only onscreen or printed on a desktop printer (like an inkjet), change the Gray pop-up menu to the Gamma choice your monitor is set to. I’ll show you how to set up your monitor in Chapter 6, “Color Management,” but for now you should know that most Macs are set to 1.8 and most Windows machines are set to 2.2.

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.

NOTES

If you’d like to measure the minimum highlight and maximum shadow settings for an output device that you own, be sure to try the highlight/shadow test that’s available on my Web site at www.digitalmastery.com/test.

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).

optimizing grayscales in CS3 photoshop tutorial

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.

NOTES

If you own a 30-bit or higher scanner and your scanning software contains a histogram and has the same adjustment controls available, you can make adjustments within your scanning software. Most scanners can deliver a histogram without gaps because they can look back to the image and pick up extra shades of gray that would fill the gaps. These days, almost all scanners are 36-bit or higher. If your scanner is capable of delivering a 16-bit grayscale image to Photoshop, the only adjustment you need to make during scanning is to make sure the highlights and shadows still have detail. If the histogram in your scanner has spikes at the ends, lower the contrast setting and rescan until you don’t get the spikes.

A Quick Levels Recap

There are several steps to using Levels to adjust grayscale images, but as I’ve said, they’re all quick and easy once you get used to them. Here’s a brief recap of the role of each of the sliders in the Levels dialog box:

  1. Move the upper-left slider until it touches the first bar on the histogram to force the darkest area of the image to black. Use the hidden Threshold feature—hold Option/Alt—to go as far as possible without damaging the image (Figures 4.25 and 4.26).

optimizing grayscales in CS3 photoshop tutorial

Figure 4.25 The original image.

optimizing grayscales in CS3 photoshop tutorial

  1. Figure 4.26 Result of adjusting upper-left slider.
  2. Move the upper-right slider until it touches the last bar on the histogram to force the brightest area of the image to white. Again, use the hidden feature to go as far as possible without damaging the image (Figure 4.27).

optimizing grayscales in CS3 photoshop tutorial

  1. Figure 4.27 Result of adjusting upper-right slider.
  2. Move the middle slider until the brightness of the image looks appropriate (Figure 4.28).

optimizing grayscales in CS3 photoshop tutorial

  1. Figure 4.28 Result of adjusting middle slider.
  2. Move the lower-left slider to make sure the shadows won’t plug up and become pure black on the printing press. Use the tables I’ve provided for settings, or ask your printer for more precise ones (Figure 4.29).

optimizing grayscales in CS3 photoshop tutorial

  1. Figure 4.29 Result of adjusting lower-left slider.
  2. Move the lower-right slider to make sure you don’t lose detail in the highlights when the smallest dots in your image disappear on the printing press. Use the tables for settings, or ask your printing company for more precise ones (Figure 4.30). I usually adjust all five sliders before clicking OK to apply them.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.30 Result of adjusting lower-right slider.

Postadjustment Analysis

Any time you adjust an image, you run the risk of introducing artifacts, so let’s take a look at what can happen to your image after applying Levels. But don’t worry—remember, there is usually at least one “fix” for every artifact.

NOTES

To see an updated histogram after adjusting the image, you must first apply the adjustment, and then reopen the Levels dialog box. You can also choose Window > Histogram to see before and after histograms overlaid on each other.

I don’t use this technique on every image, just on those that have extremely noticeable posterization.

If you don’t have the time or patience to apply the Eliminating Posterization technique mentioned here, consider choosing Filter > Noise > Add Noise and use a setting of 3 or 4. That can help to reduce posterization, but will not be able to help in cases of extreme posterization.

Low Contrast Onscreen Appearance

If you’ve adjusted an image that will eventually be reproduced on a commercial printing press, your results will most likely look rather flat onscreen (lacking contrast). This problem is a temporary one since the image will gain contrast when it’s printed on press (dark areas become darker and bright areas become brighter). You’re welcome to adjust the top three sliders in Levels to get an acceptable image and then hold off on adjusting the bottom two sliders until you’re done working on the image in Photoshop. That way, the image will have good contrast for the vast majority of the time you work on it and then the bottom two sliders can be adjusted right before saving the image so that it’s ready to be reproduced on press.

Recognizing Posterization

When you look at an updated histogram, you might see wide gaps in the histogram—this indicates posterization (Figure 4.31). Posterization is when you should have a smooth transition between areas and instead you see a drastic jump between a bright and dark area. Some call this banding or stair-stepping. As long as the gaps in the histogram are smaller than three pixels wide, you probably won’t notice it at all in the image.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.31 Gaps in a histogram indicate posterization.

Adjusting the image usually causes these gaps. As you adjust the image, the bars on the histogram spread out and gaps start to appear (remember that Slinky). The more extreme the adjustment you make, the wider the gaps. And if you see those huge gaps in the histogram, it’ll probably mean that the posterization is noticeable enough that you’ll want to fix it (it usually shows up in the dark areas of the image).

Eliminating Posterization

Here’s a trick that can minimize the posterization. I should warn you that you have to apply this technique manually to each area that is posterized. Although it might take you a little bit of time, the results will be worth it.

To begin, select the Magic Wand tool, set the Tolerance to 0, and click on an area that looks posterized. Next, choose Select > Modify > Border, and use a setting of 2 for slight posterization or 4 for a moderate amount of posterization. Now apply Filter > Blur > Gaussian Blur until the area looks smooth (Figures 4.32 and 4.33). Repeat this process on all of the posterized areas until you’re satisfied with the results.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.32 Turn off the Preview check box to see the edges of the posterized area.

Figure 4.33 With the Preview check box turned on, increase the Radius setting until the posterized area appears smooth.

If you find that a large number of your images end up with post-scan posterization, you might want to look into getting a scanner that’s capable of delivering 16-bit images to Photoshop. A typical grayscale image contains no more than 256 shades of gray, which is technically known as an 8-bit image. That’s sufficient for most images, but extreme adjustments will cause posterization. One way to avoid posterization is to use a scanner that can produce images that contain thousands of shades of gray, which is technically known as a 16-bit image. Most scanners are capable of capturing more than 256 shades of gray from a photograph, but few are capable of actually delivering all those shades to Photoshop. So, the next time you shop for a scanner, be sure to ask if it is capable of delivering 16-bit images to Photoshop.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.30 Result of adjusting lower-right slider.

Postadjustment Analysis

Any time you adjust an image, you run the risk of introducing artifacts, so let’s take a look at what can happen to your image after applying Levels. But don’t worry—remember, there is usually at least one “fix” for every artifact.

NOTES

To see an updated histogram after adjusting the image, you must first apply the adjustment, and then reopen the Levels dialog box. You can also choose Window > Histogram to see before and after histograms overlaid on each other.

I don’t use this technique on every image, just on those that have extremely noticeable posterization.

If you don’t have the time or patience to apply the Eliminating Posterization technique mentioned here, consider choosing Filter > Noise > Add Noise and use a setting of 3 or 4. That can help to reduce posterization, but will not be able to help in cases of extreme posterization.

Low Contrast Onscreen Appearance

If you’ve adjusted an image that will eventually be reproduced on a commercial printing press, your results will most likely look rather flat onscreen (lacking contrast). This problem is a temporary one since the image will gain contrast when it’s printed on press (dark areas become darker and bright areas become brighter). You’re welcome to adjust the top three sliders in Levels to get an acceptable image and then hold off on adjusting the bottom two sliders until you’re done working on the image in Photoshop. That way, the image will have good contrast for the vast majority of the time you work on it and then the bottom two sliders can be adjusted right before saving the image so that it’s ready to be reproduced on press.

Recognizing Posterization

When you look at an updated histogram, you might see wide gaps in the histogram—this indicates posterization (Figure 4.31). Posterization is when you should have a smooth transition between areas and instead you see a drastic jump between a bright and dark area. Some call this banding or stair-stepping. As long as the gaps in the histogram are smaller than three pixels wide, you probably won’t notice it at all in the image.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.31 Gaps in a histogram indicate posterization.

Adjusting the image usually causes these gaps. As you adjust the image, the bars on the histogram spread out and gaps start to appear (remember that Slinky). The more extreme the adjustment you make, the wider the gaps. And if you see those huge gaps in the histogram, it’ll probably mean that the posterization is noticeable enough that you’ll want to fix it (it usually shows up in the dark areas of the image).

Eliminating Posterization

Here’s a trick that can minimize the posterization. I should warn you that you have to apply this technique manually to each area that is posterized. Although it might take you a little bit of time, the results will be worth it.

To begin, select the Magic Wand tool, set the Tolerance to 0, and click on an area that looks posterized. Next, choose Select > Modify > Border, and use a setting of 2 for slight posterization or 4 for a moderate amount of posterization. Now apply Filter > Blur > Gaussian Blur until the area looks smooth (Figures 4.32 and 4.33). Repeat this process on all of the posterized areas until you’re satisfied with the results.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.32 Turn off the Preview check box to see the edges of the posterized area.

optimizing grayscales in CS3 photoshop tutorial

Figure 4.33 With the Preview check box turned on, increase the Radius setting until the posterized area appears smooth.

If you find that a large number of your images end up with post-scan posterization, you might want to look into getting a scanner that’s capable of delivering 16-bit images to Photoshop. A typical grayscale image contains no more than 256 shades of gray, which is technically known as an 8-bit image. That’s sufficient for most images, but extreme adjustments will cause posterization. One way to avoid posterization is to use a scanner that can produce images that contain thousands of shades of gray, which is technically known as a 16-bit image. Most scanners are capable of capturing more than 256 shades of gray from a photograph, but few are capable of actually delivering all those shades to Photoshop. So, the next time you shop for a scanner, be sure to ask if it is capable of delivering 16-bit images to Photoshop.

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