Resolving Resolution Issues


Although the Resolution text box is positioned unceremoniously toward the bottom of the Image Size dialog box as shown in Figure 4-3, it’s one of the most critical values to consider if you want your images to look good.

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Figure 4-3: You control file size, image dimensions, and resolution through the Image Size dialog box.

The Resolution value determines how tightly the pixels are packed together when printed. It’s kind of like the population density of one of those ridiculously large urban areas cropping up all over the modern world. Consider Lagos, Nigeria, which is a city of nearly 10 million souls — more than London, Paris, or Shanghai — and continues to grow at a rate of 5 percent each year. Although population explosions in urban environments can have some negative impacts, the larger the number of pixels you have in an image, the better the print quality.

Remember

The rule is simple: More pixels means higher resolution and better print quality. On the other hand, having more pixels also means larger file sizes, which can bog down the transfer of images via electronic formats such as e-mail.

In order to increase pixel population density (thereby improving resolution), you have to either increase the number of people in a city or decrease the physical boundaries of the city and scrunch everyone closer together. If you want a higher resolution (more pixels per inch), you can either decrease the physical dimensions of the image or increase the file size (pixel dimensions) by adding pixels to the image. For example, the two images in Figure 4-4 have the same file size, but the smaller image has twice the resolution of the larger image — 180 pixels per inch versus 90 ppi.

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Figure 4-4: Two images with the exact same number of pixels but subject to two different resolutions.

Likewise, when you increase the dimensions of an image or delete some of its pixels, the resolution goes down. Here are some things you ought to know about resolution:

  • Unlike the population density of a city resolution is constant across the board. It is equally dense at all points.

  • The boundaries of images are always rectangular.

  • When you increase or decrease resolution of an image, the image gets bigger or smaller, but it stays rectangular — it grows or becomes smaller.

  • Population density is measured in terms of area — you know, so many folks per square mile. Resolution, on the other hand, is measured in a line — pixels per linear inch. So an image with a resolution of 180 pixels per inch contains 32,400 pixels per square inch. (That’s 180 2.)

Changing pixel dimensions

The top two text boxes (the Width text box and the Height text box, which, lumped together, are referred to as Pixel Dimensions) in the Image Size dialog box (refer to Figure 4-4) enable you to change an image’s pixel dimensions — the number of pixels in the image. Unless you want to risk ruining your image — or you really, really know what you’re doing when it comes to pixels — avoid these text boxes like the plague.

If changing the pixel dimensions is so dangerous, you may wonder why Photoshop gives you the option to do so at all. Well, although I don’t recommend ever adding pixels to an image, you may need to lower the pixel dimensions on occasion. If your file size is really large — that is, your image contains a ton of pixels — you may want to toss some of the pixels overboard. See the following section, “Dumping pixels,” for more information.

Technical Stuff

In some particularly nerdy circles, changing the number of pixels in an image is called resampling. The idea is that you sample the photograph when you scan it — as if that makes a lick of sense — so any adjustment to the quantity of pixels after scanning is resampling. Photoshop uses the term resampling, but I prefer to call it resizing, because this gives folks a fighting chance of understanding what I’m talking about.

Dumping pixels

When you delete pixels, you delete detail. Figure 4-5 shows what I mean. The physical size of all three images is the same, but the detail drops off from one image to the next. The top image contains 64,000 pixels and is printed at a resolution of 140 ppi; the middle image contains 1/4 as many pixels and is printed at 70 ppi. The bottom image contains only 4,000 pixels and has a resolution of 35 ppi. Notice how details such as the shadows from the girl’s eyelashes and the distinction between individual hairs in her eyebrows become less pronounced and more generalized as the pixel population decreases.

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Figure 4-5: Three images, each containing fewer pixels and printed at a lower resolution

Warning

Lowering the Pixel Dimensions values can be dangerous because what you’re really doing is throwing away pixels. Because pixels = details, tread carefully. When pixels are deleted, you can never get them back.

In an ideal world, you’d want as many pixels as possible because the more pixels you have the greater the image detail. But the more pixels it has the more disk space an image consumes, which can be a problem if you’re working with limited computing resources. For one thing, large file sizes can slow down Photoshop substantially. Also, if you’re preparing your image for the Web, you need to reduce your file size and/or your resolution so that your image displays properly and loads quickly in the Web browser.

Remember

Even when you dump pixels from an image, however, you shouldn’t attack the job from the Pixel Dimensions text boxes; I show you a better way in the steps in the section, “Using the Image Size dialog box.”

Adding pixels

Increasing the file size (by raising the Pixel Dimensions values) isn’t such a hot idea because Photoshop can’t generate image elements (pixels) out of thin air. When you raise the Pixel Dimensions values, Photoshop adds pixels by averaging the color of the preexisting pixels (a process computer nerds call interpolation) in a way that may result in image softening. Circumstances don’t often arise in which the softening of detail is desirable, and I can guarantee that there will never be an occasion in which interpolation results in the miraculous reconstruction of detail.

Changing the physical dimensions of an image

The Width and Height boxes in the Document Size portion of the Image Size dialog box reflect the actual printed size of your image and the approximate size of your image when distributed over the Web. (Because users have different sized monitors with different display settings, the actual size of the image may change a little when viewed on different monitors.)

The drop-down list boxes next to the Width and Height options let you change the unit of measure displayed in the text boxes. For example, if you select picas from the Document Size Width drop-down list box, Photoshop converts the Width value from inches to picas. (A pica is an obscure typesetting measurement equal to 1/6 inch.) The percent option in the drop-down list box enables you to enter new Width and Height values as a percentage of the original values. Enter a value higher than 100% to increase the print size; enter a value lower than 100% to reduce the print size.

Warning

When you change the document size of the image, either the Resolution value or the number of pixels in the image automatically changes, too, which can affect the quality of your image. For more information, read the “Changing Pixel Dimensions” section earlier in this chapter. And for details on how to change the print size without ruining your image, see the “Using the Image Size dialog box safely” section later in this chapter.

Type of Job

Ideal Resolution

Acceptable Setting

Full-color image for magazine or professional publication

300 ppi

225 ppi

Full-color slides

300 ppi

200 ppi

Color ink jet printers

300 ppi

200 ppi

Color images for laser printing or overhead projections

180 ppi

120 ppi

Color images for multi- media productions and Web pages

72 ppi

72 ppi

Black-and-white images for image-set newsletters, flyers, and so on

180 ppi

120 ppi

Black-and-white images for laser printing

90 ppi

Keeping things proportionate

Both pairs of Width and Height text boxes in the Image Size dialog box list the dimensions of your image in the current unit of measure. If you enter a different value into either text box and click on the OK button (or press Enter), Photoshop resizes your image to the dimensions. Pretty obvious, eh?

start sidebar
Yeah, okay, but what resolution should I use?
Tip

The Auto button in the Image Size dialog box is supposed to generate a perfect resolution value based on the line screen that your printer will use. The only problem is that no one knows what a line screen setting is. Rather than bother with trying to explain this arcane bit of printing technology to you at this point I decided to come up with both ideal and acceptable values for certain kinds of print jobs.

end sidebar

But strangely, when you change either the Width or Height value, the other value changes, too. Are these twins that were separated at birth? Is there some new cosmic relationship between Width and Height that’s known only to outer-space aliens and the checkout clerk at your local grocery store? No, it’s nothing more than a function of the Constrain Proportions check box, which is turned on by default. Photoshop is simply maintaining the original proportions of the image.

When you click to turn off the Constrain Proportions option, Photoshop permits you to adjust the Width and Height values independently. Notice that the little link icon (labeled back in Figure 4-3) disappears, showing that the two options are now maverick independents with reckless disregard for one another. You can now create stretchy effects like the ones shown in Figure 4-6. In the top example, I reduced the Width value by a factor of two and left the Height value unchanged. In the bottom example, I did the opposite, reducing the Height value and leaving the Width value unaltered.

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Figure 4-6: Known to friends and family as Kid Squishums, this versatile little tyke is the result of deselecting the Constrain Proportions check box.

Warning

In order to deselect the Constrain Proportions check box, you have to select the Resample Image check box. As explained later, in the section “Using the Image Size dialog box safely,” when the Resample Image check box is selected, Photoshop either adds or deletes pixels from your image to compensate for the changes you make to the width and height of the image. Because adding pixels can make your image look like mud, never increase the width or height value with Constrain Proportions deselected. Decreasing the width and height values is okay, as long as the Resolution value stays in the acceptable range. (See the sidebar “Yeah, okay, but what resolution should I use?” earlier in this chapter for recommended resolution values.)




Photoshop CS For Dummies
Photoshop CS For Dummies
ISBN: 0764543563
EAN: 2147483647
Year: 2006
Pages: 221

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