Solution with Levels

Levels 

The Levels dialog box (choose Image > Adjustments > Levels) is the cure for most common complaints about grayscale image quality. It offers you far more control and feedback than Brightness/Contrast. Instead of having only two sliders to adjust, Levels offers you five, as well as a bar chart that indicates exactly what is happening to the image. And unlike the sliders in Brightness/Contrast, the Levels sliders don't change the entire image in equal amounts.

It might take several pages to describe all the controls in the Levels dialog box, but once you know how to use them, it will take you less than a minute to optimize an image. Just remember to apply all of the controls in Levels, because each builds on the last. You can liken the steps in Levels to the ingredients in crème brulée - leave one out and you might end up with pudding instead of perfection.

Note: 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 space the shade takes up—how much it's 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.

The Histogram (Bar Chart)

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 directly below the first bar that appears on the left end of the histogram, you can determine the darkest shade of gray in the image. If there were anything darker than that, then there would have to be some bars above those shades in the histogram. By looking directly below the last bar that appears on the right end of the histogram, you can determine the brightest shade of gray in the image. So if you look at the Figure below, 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%.

There is no ideal when it comes to a histogram; it's simply a reflection of which shades of gray are most prevalent in your image. Tall bars indicate a shade of gray that takes up a lot of space in the image, and short bars 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.

Adjusting Contrast

Note: 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.

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.

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 will 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 be using the full range of shades available to a grayscale image. 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.

Other Important Features

To achieve maximum contrast without sacrificing detail, Adobe created a hidden feature in the Levels dialog box. It's known as Threshold mode because it acts like the Threshold dialog box that we used in Bonus Chapter 1, "Line Art Scanning" on the CD at the back of this book. 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 key (Mac) or Alt key (Windows) when you move the upper-right or upper-left sliders in the Levels dialog box.

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.

Note: 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.