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Using the Image Size Dialog

The Image Size dialog is always a big source of confusion, because the results you get depend not only on which buttons you click and which fields you type numbers into, but also on the order in which you do so. The Image Size dialog is split into the two sets of numbers that determine an image’s resolution: its pixel dimensions, and its size if it’s printed.

Pixel Dimensions. The best way to specify an image’s size is by its pixel dimensions—these tell you exactly how much data you have to work with. The Pixel Dimensions section shows you both the dimensions and the file’s size, in megabytes (or kilobytes, if it’s under 1 MB).

Document Size. A bitmapped image has no inherent size; it’s just pixels on a grid. The Document Size section lets you state a size and resolution, so that when you import the file into some other program, it knows what the image’s size will be when it’s printed.

The Resample Image check box affects resizing in a fundamental way. When you turn this option on, Photoshop lets you change the image’s pixel dimensions; when it’s off, the pixel dimensions are locked. Leave Resample Image off if you don’t want to increase (upsample) or decrease (downsample) the number of pixels that make up the image. The Resample Image pop-up menu at the bottom of the dialog becomes available when Resample Image is checked. For more information about how these options affect an image, see “Resampling,” later in this chapter.

Setting Size Options in the Right Sequence

One point of potential confusion is that whenever you make a change to one field in the Image Size dialog, some fields change and others don’t. Sometimes editing a value wipes out another value you entered earlier! Without an understanding of how the Image Size options interact, you can sometimes feel like a frustrated safecracker. There actually is logic behind it; here’s our quick cheat sheet to getting what you want (see Figure 2-9):

  • To lock down the current file size and the current number of pixels in the image, turn off Resample Image. Now edit the Document Size as needed. When Resample Image is off, changing either Resolution or Width and Height always changes the other option so that the file size and total number of pixels remain constant.
  • To “up-res” (upsample) for large-format prints, turn on Resample Image and choose Bicubic Smoother from the Resample Image pop-up menu. Then enter the Document Size and Resolution appropriate for the print size and printer you’re using. Don’t touch Pixel Dimensions.
  • To resize an image for print while locking down the current resolution, turn on Resample Image and pick an appropriate option from the Resample Image pop-up menu. Then change the Document Size Height and Width as needed. Don’t change anything else.
  • To resample an image for the Web or video, turn on Resample Image and pick an appropriate option from the Resample Image pop-up menu. Now enter the Pixel Dimensions required by your project, but don’t touch anything else. We mean it—editing anything in the Document Size section will screw up the Pixel Dimensions values.

Figure 2-9 How the Image Size dialog works

Scaling by Percentage

The word percent appears in both the Pixel Dimensions and the Document Size pop-up menus. Percent isn’t an absolute size; it’s based on the current size of the image you’re working on. For example, if you have a 2-by-2-inch image and you type in “200 percent” for Document Size Width and Height, the result will be a 4-by-4-inch image. The resulting number of pixels in the image depends on whether Resample Image is on or off.

We find this especially helpful when we have to re-create an image that was scaled “for position only” in a layout program. First we write down the scaling values on a piece of paper, and then we open the image in Photoshop and type the percentages into the Image Size dialog.

The best way to learn how the Image Size dialog works, though, is to open an image, note the starting values, and play around with it. So get going!

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