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Printed Images

So far, we've talked about scanning resolution (spi) and image resolution (ppi), but you might find that many people use the term dots per inch, or dpi to describe resolution. Don't let them confuse you! Dpi has nothing to do with scanning or displaying an image. Instead, it describes the size of the dots that your printer uses to output an image. There is an important difference between dots and pixels, which explains why images are measured in pixels per inch instead of dots per inch. Dots can only have one of two states—on or off. A photograph printed on a desktop printer might look as if it contains shades of gray, but if you were to look at it under a magnifying glass, you'd see that it's really made up of pure black dots (Figure 4.3). A color printer is no different; it has four colors of ink (cyan, magenta, yellow, and black), but it still prints with solid dots of those colors (Figure 4.4). In order to simulate a shade of gray (or a shade of color, for that matter), the printer uses a pattern of black, or solid-colored, dots (different types of printers will use different kinds of patterns, as you'll see later in this chapter). Packing black dots together in a very dense pattern will create an area that looks like a dark shade of gray, and spacing them out will create what looks like a lighter shade. As long as the colored dots your printer uses are small enough that your eye can't focus on them individually, your brain is fooled into thinking that you are seeing shades of gray or shades of color (Figure 4.5), when you are really looking at solid black (or colored) dots.

Figure 4.3. When you look at a printed image, your eye sees shades of gray, when in reality your image is printed using solid black dots. (©1998 PhotoSpin, www.photospin.com)


Figure 4.4. Color images are printed using solid cyan, magenta, yellow, and black dots.


Figure 4.5. If the dots your printer uses are small enough that your eye can't focus on them, then your brain will be fooled into thinking that you're looking at shades of gray, when in reality those shades are a pattern of solid black dots.


Your printer needs to use more than one dot to create the pattern necessary to simulate a shade of gray, which means the pixels that make up your image should always be larger than the dots your printer uses to output them. In essence that means the resolution of your image (ppi) should always be lower than the resolution of your printer (dpi).

Note

Printer resolution isn't always measured in dots per inch (dpi). A dye-sub printer produces output that resembles a photographic print because it can create true shades of gray. Since dye-sub printers don't use pure black and pure white dots, their resolution is measured in pixels per inch (ppi).


A popular misconception is that higher image resolution settings are better, but that's simply not true. Your printer can handle only so much information, and when you feed it too much, your file size ends up being considerably larger than is needed, and the image will take a long time to print. What's happening is that the printer will discard all the extra information that it can't reproduce, and in doing so will also cause the quality of your image to suffer. As it attempts to discard the extra information, it will take the sharpening that was originally applied to your image and just average it out.

When the image resolution setting is too low, the pixels will be so big that you can easily see their edges, which will make the image appear pixilated (just like when you try to print an image from the Web). If you want the highest quality possible from your output device, then you need a resolution setting that is optimal for whatever kind of output device you use.

There are a few special instances when the resolution of your image (ppi) should match the resolution of your printer (dpi). For example, when your image is made out of pure black and pure white pixels (logos, text, and so on), it is a good idea to make those pixels the same size as the dots your printer will use to output the image. That way you'll get the highest-quality, most jaggy-free image possible. Another instance is when your output device can truly deliver shades of gray instead of simulating them with pure black dots. That is the case with dye-sub printers and 35 mm slides.

To recap:

  • Samples Per Inch (spi) is how many pixels your scanner will create out of each inch of the original image.

  • Pixels Per Inch (ppi) is how large the pixels of an image will be when printed.

  • Dots Per Inch (dpi) describes the size of the dots your printer uses to reproduce your image.

It's nice to be familiar with these terms, but don't feel that you have to be an expert in defining them. So many people interchange them indiscriminately (even scanner manufacturers will label the spi setting as dpi) that it's usually better to avoid them altogether and just think about the device you're using. Scanning resolution might be measured in spi, but I just call it scanning resolution. Image resolution might be measured in ppi, but I just say my image has a resolution of 300, and that way anyone who misuses the terms will know what I mean. The one situation where I use the proper term is when describing the resolution of my printer (dpi). That's because everyone seems to get that one right, so it can't be easily confused.

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