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Scanning

When you scan an image, the resolution setting determines the width and height of the image, measured in pixels. In scanning, resolution is measured in samples per inch, or spi. The term makes sense because your scanner is sampling the image—seeing what it looks like—a set number of times in an inch. Scan a 3-by-5-inch original at a resolution of 100 spi, and you will end up with exactly 300 by 500 pixels. Or scan the same image at 72 spi, and you'll get 216 by 360 pixels total (3 inches x 72 samples per inch = 216, and 5 inches x 72 samples per inch = 360). Simple math will determine how many pixels you end up with.

But if that's the case, then it would seem that you could call that pixels per inch (ppi), because that's what you're ending up with, right? Almost, but not exactly. There is something called pixels per inch, but it has nothing to do with scanning. Instead, it indicates how large the pixels will be when you print the image. If you reproduce your image at 100% of its original size, then the spi and ppi settings are identical. But what if you scale your image up to twice its original size? In that case, you might take 200 samples per inch when the image is scanned, and then print the image at 100 pixels per inch. To understand this better, think back to that 3-by-5-inch image. If you scan it at a resolution of 200 samples per inch, then you'll end up with 600 by 1000 pixels. Then if you print those same pixels, but only print 100 of them per inch, wouldn't you end up with a 6-by-10-inch printed image? Again, it's simple math (600 pixels divided by 100 per inch = 6 inches). So, you could think of the spi setting as the scanning resolution and the ppi setting as the image's resolution. They are directly related to each other, but not the same.

In addition to understanding how resolution works, you also need to know that scanner manufacturers are doing their part to throw a curveball into the equation: some scanners offer a scale setting and others don't. This can put a lot of unnecessary guesswork into the process. If your scanner has a scale setting, then it will do a lot of work behind the scenes and make your life much easier.

Scanners with a scale setting use the resolution setting to determine how large you'd like the pixels to be when they are printed (also known as the ppi setting). Then, to determine how many pixels need to be captured from the original, the scanner takes the resolution setting (let's say you entered 100) and multiplies it by the scale setting you entered (let's say you used 200%). This determines the scanning resolution, which is also known as the spi setting. In the above example, your scanner would take 100 pixels per inch and multiply it by 200% to end up with a scanning resolution of 200 samples per inch. Let's look at what you'll have to do if your scanner doesn't offer a scale setting, and then you'll see why I like scanners that offer one.

With no scale setting, you are forced to calculate the scanning resolution yourself. To make that calculation, you have to do two things. First, take the image resolution you want to end up with; for now, let's say you need 300 pixels per inch. (I'll show you how to figure out what image resolution you want once we start talking about viewing the image onscreen and printing it.) Then multiply the resolution by how much you'd like to scale your image; in this case, let's make it 33%. If you want the image to be half its original size, then multiply the resolution by .5 or (50%); for one-third of its original size, multiply it by .3333 (or 33%). Then use the result as your scanning resolution. In this case, 300 multiplied by .3333 gives me a scanning resolution of 100. But I'm not done yet. If I open that scanned image in Photoshop and choose Image > Image Size, the size and resolution won't be what I really want. Remember, I want an image that is 33% of its original size with a resolution of 300. But I've done nothing to inform my scanner, or Photoshop, of my intentions. That means Photoshop only has the information my scanner supplied, which is that the image was scanned with a resolution of 100, with no scaling (remember, for this example the scanner does not have a scale setting).

That means we have one important step left. If you look at the top of the Image Size dialog box, you'll see how many pixels make up the image. That should be the exact amount we need, but the resolution setting is not what we want it to be (Figure 4.1). To fix that, uncheck the Resample Image checkbox, and then enter the resolution you want (300 in this case) as shown in Figure 4.2. Photoshop will use simple math (width in pixels divided by resolution setting) to figure out how large the image will be when printed. As you'll find out later in this chapter, resampling is similar to rescanning an image with a different resolution setting. In our example, we have the proper amount of information, but the pixels would be too large when printed, so we don't need to resample the image. Alas, the world would be a better place if scanner manufacturers would just include a scale setting, but sadly, many don't.

Figure 4.1. The Image Size dialog box indicates the settings that were used when the image was scanned.


Figure 4.2. Unchecking the Resample Image checkbox allows you to change how large the image will be when printed, without changing the number of pixels that make up the image.


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