Setting Image Size, Resolution, and Document Size

Image size, resolution, and document size form a trinity of factors that are involved in a relationship. Changes to any of these three factors affect the others. You manage this relationship from the Image Size dialog box.

You will open a sample file to explore the Image Size command. Open the file Lunch.psd from the CD.

Choose Image > Image Size to open the Image Size dialog box.

To begin with, make sure the Resample Image checkbox is unchecked. While this setting is off, the relationship mentioned earlier is made simpler.

Note that the Document Size is currently more than 11 by 8 inches while the resolution is 72 pixels per inch. Change the resolution to 200. The width and height automatically adjust to 4 by 3 inches.

NOTE Traditionally screen resolution is either 72 or 96 pixels per inch (ppi). Most web design work is done at 72 ppi.

5. Change Width from 4 inches to 8. As you do this, the other two parameters (Height and Resolution), automatically change, so the image now measures 8 by 6 inches with a resolution of 100 pixels per inch.

NOTE Resolution and document size are inversely proportional; you trade one for the other.

6. Place a check in the Resample Image check box. This enables the 2-pixel dimension parameters (see Figure 1.45).

7. Change Width in the Pixel Dimensions group to 1000. As you do this, notice that the document size has increased (10 by 7.5 inches) while the resolution remains constant. Also note that the file size has increased because more pixels must now be stored.

8. Click the Cancel button in the Image Size dialog box to discard the changes you made while experimenting.

Figure 1.45

The Image Size dialog box

This check box enables the Pixel Dimensions boxes.

WARNING Resampling an image to much larger pixel dimensions does not increase the quality of the image. The extra pixels are merely resampled and interpolated from the existing pixels. If you really want a much larger, higher-quality image, you must start with an image of higher resolution. If you are using a digital camera, for example, you must use a camera with more megapixels.

When you start with more pixels in an image (by using a 5-megapixel camera, for example), you can make larger prints of a high quality. Less-expensive digital cameras (2 megapixels, for example) cannot produce high-quality prints of the same size because less information is captured from the real world.

TIP A rule for reasonable resolution is not to print images at less than 200 pixels per inch; 72 pixels per inch is sufficient for display on a computer monitor.

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