Changing Print Dimensions and Resolution in Adobe Photoshop CS5
From the book
From the book
Changing the document resolution and dimensions
In this section, you’ll encounter three related terms:
- A file’s pixel count (or pixel dimensions) is arrived at by multiplying its pixel height and width (as in 3000 × 2000 pixels).
- The resolution (or “res,” for short), is the fineness of detail in a document, and is measured in pixels per inch (as in 250 or 300 ppi).
- The process of changing a file’s pixel count (adding or deleting pixels) is known as resampling.
Some input devices (e.g., digital cameras that capture 8 megapixels of data or more and high-end scanners) produce files with a higher pixel count than is needed for most standard printing devices. In Photoshop, you can take advantage of a file’s high pixel count to increase its print size or print resolution. You can keep the pixel count constant as you increase the print size (and thereby lower the resolution) or increase the resolution (and thereby lower the print size). No resampling occurs in either case, so the image quality isn’t diminished.
You will need to resample a file if it contains too few or too many pixels to meet the resolution requirement of your target output device. If you resample a file as you increase its resolution, pixels will be added to it and its storage size will increase accordingly. Resample a file as you decrease its resolution (downsample it), and pixels will be deleted. The only way to get those pixels back is by clicking a prior state on the History panel before closing the file. Even more important, resampling reduces the image clarity. This can be a problem for print output, depending on the output resolution and how drastically the file is resampled, although it can be remedied somewhat by applying a sharpening filter afterward (see pages 296–300). Resampling isn’t a problem for Web output.
We’ll show you how to resize three common types of files for print output — low res/large dimensions, high res/small dimensions, and medium res/small dimensions — and how to resize a file for Web output. By default, photographs from a digital SLR camera have a low resolution (72 to 180 ppi) and very large width and height dimensions. They contain a sufficient number of pixels for high-quality output (prints as large as 8″ × 12″), provided you increase their resolution to the proper value. You can do this via the Image Size command in Photoshop.
To change the resolution of a digital photo for print output (low res/large dimensions)
- With the file open in Photoshop, choose Image > Image Size (Ctrl-Alt-I/Cmd-Option-I).
- The Image Size dialog opens.A Because you need to increase the image resolution, uncheck Resample Image. When you lower the Width and Height in step 4, the resolution will increase automatically.
- In the Document Size area, choose a unit of measure from the menu next to the Width field (we chose inches); the same unit will be chosen automatically for the Height.
- Enter the Width or Height for the desired print size; the Resolution value increases.
- If the resolution is now between 240 and 300 pixels per inch, you’ve achieved your goal — just click OK.B The pixel dimensions didn’t change, so you won’t need to resharpen the image (A–B, next page).
If the resolution is greater than 300 ppi, check Resample Image, then enter a Resolution of 300. Also, from the menu at the bottom of the dialog, choose a resampling method for the way in which Photoshop reassigns color values based on the values of existing pixels. The Bicubic (Best for Smooth Gradients), Bicubic Smoother (Best for Enlargement), and Bicubic Sharper (Best for Reduction) options cause the least reduction in image quality. As a result of resampling, the pixel dimensions of the image will have changed.
- Click OK. If the image was resampled, you should now resharpen it (see pages 296–300).
- To restore the settings that were in place when you opened the Image Size dialog, Alt-click/Option-click Reset (the Cancel button becomes a Reset button).
- To specify a default Image Interpolation method for Photoshop features, such as the Image Size dialog, see page 386.
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In many cases, scanned images have a high resolution and small dimensions and contain a sufficient number of pixels for large printouts.
To resize a scanned image for print output (high res/small dimensions)
- Choose Image > Image Size (Ctrl-Alt-I/Cmd-Option-I). The Image Size dialog opens.
- Make sure Resample Image is unchecked.
- Increase the Width or Height to the size needed for your printout. The Resolution will decrease.
If the Resolution falls between 240 and 300 ppi, you’re done; click OK. Because no resampling occurred, no resharpening is necessary.
If the Resolution is still greater than 300 ppi, check Resample Image,C then lower the Resolution to 300. From the menu at the bottom of the dialog, choose Bicubic Smoother (Best for Enlargement) as the interpolation method. You’ve just resampled the image, so you should resharpen it after clicking OK (see pages 296–300).
- Click OK.
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Small files (with a resolution of, say, 180 to 200 ppi) lack a sufficient number of pixels to be enlarged without resampling, so they must be resampled to achieve the dimensions needed for print output. This is not an ideal scenario, as it reduces the image sharpness, and you’ll certainly need to apply a sharpening filter afterward.
To resize a scanned image for print output (medium res/small dimensions)
- Choose Image > Image Size (Ctrl-Alt-I/Cmd-Option-I). The Image Size dialog opens.
- Check both Resample Image and Constrain Proportions.
- Enter the Width desired for your printout. The Height value will change proportionately and the file storage size and pixel dimensions will increase.
- Click OK. Since the image was resampled, you should now use a sharpening filter to resharpen it (see pages 296–300).
Because Web images are viewed on computer displays, which are low-resolution devices, they should have a lower pixel count than images designed for print output. In most cases, you will need to downsample your files (discard image pixels) to make them the correct size for output.
To change the pixel dimensions of an image for Web output
- Use File > Save As to make a copy of your file, then choose Image > Image Size (Ctrl-Alt-I/Cmd-Option-I). The Image Size dialog opens.A
- Make sure Resample Image is checked.
- To preserve the width-to-height ratio of the image, check Constrain Proportions.
- From the menu at the bottom of the dialog, choose the Bicubic Sharper (Best for Reduction) resampling method, which will degrade the image the least.
- Enter a Resolution of 72 ppi.
- In the Pixel Dimensions area, choose pixels from the menu (the default unit), then enter the exact Width and/or Height dimensions needed.B
- Click OK. On pages 421–426, you will learn how to optimize Photoshop files for output to the Web.
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Pixel Counts, Resolution, and Image Size Compared
The illustrations below explain the concept of resolution and how it affects image size. Figures A–B compare the same image at two different resolutions, and figures C–D compare the print sizes for those resolutions.
The moral here: Don’t judge the output size of an image based on its onscreen size. Instead, consider these two factors: the current zoom level of the image in the document window and the image resolution.
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