Understanding Photoshop is a biweekly column that takes an in-depth look at how digital photographs are built and manipulated. It is a college-level course in plain English for free at Photofocus. To learn more see this article.
Once you’ve acquired your digital images, you’ll need to size them for your project (as well as ultimate output). For many Photoshop users, such as photographers, this may be as straightforward as cropping and sizing. In this part of our series, let’s explore several techniques for sizing your images. You’ll learn about the concept of resampling, which addresses how the computer adds or subtracts information from a digital image while trying to retain detail and clarity.
This photo was scanned at two different resolutions. The image on the left was scanned at 300 spi, and the image on the right was scanned at 72 spi. Examine the detailed enlargements to see the impact of different scanner settings.
Earlier we looked closely at the process of acquiring digital images. If you skipped ahead or missed it, go back—a solid understanding of those concepts is required to move forward. Quite simply, you must know the capabilities of your digital camera or scanner to process information.
We also briefly discussed resolution requirements for different output formats. The second part of the image-sizing puzzle is a clear understanding of these output requirements. What resolution does your printer need? Are you sending the image to a service provider such as a commercial printer? You’ll need to make lots of choices, but they should be based on where the image needs to end up. Know the destination of your image so you’ll know which path to take.
TIP: Start Out Right: Digital Cameras
If you’re acquiring a digital image, be sure to capture enough pixels. If you want an 8 × 10 inch print and need 300 dpi, do the math before shooting. Multiply the inch size by the print resolution. In this example: 8 × 300 = 2400 and 10 × 300 = 3000. Therefore, 2400 × 3000 = 7,200,000, which is about 7.2 megapixels. To allow for cropping, you’ll want to shoot at an even higher resolution.
The process of resampling allows you to change the pixel dimensions of your image. This will affect the display and print size of your image. This part of the resizing process is important for several reasons:
- Images will print faster when they are sized properly for your output device.
- Images will print clearer when you size them to a target size and then run a sharpening filter to enhance the edge detail.
- Images appear crisper when they are displayed at 100 percent on a computer screen (such as for a PowerPoint presentation or website).
The process of resampling is often identified based on whether you are scaling the image smaller (downsampling) or larger (upsampling):
- Downsampling. If you decrease the number of pixels in an image, you are downsampling the image, which permanently discards data. You can specify an interpolation method (discussed in the next section) to determine how pixels are deleted.
- Upsampling. When upsampling, you create new pixels to expand the image. Again, you can specify an interpolation method to determine how pixels are added. When upsampling, you add information that did not previously exist, which generally just makes a larger image that may appear less sharp than the original.
Choose an Interpolation method
When you resample an image, Photoshop creates new pixels. Those new pixels are created based on the neighboring pixels. How those new pixels are formed is determined by the interpolation method you specify. Photoshop offers up to five methods to resample your image.
Choose one of the following methods:
- Automatic. This option automatically switches between the three bicubic methods based on the task at hand. For most users, this is the best option to choose.
- Preserve Details (enlargement). This option makes it easy to upscale images as it provides a Noise reduction slider for smoothing out noise as you upscale the image.
- Bicubic Smoother (enlargement). This method is a refinement of Bicubic. It is specifically designed for upsampling (enlarging images).
- Bicubic Sharper. (reduction) This method is also a refinement of Bicubic. It is useful for downsampling (shrinking images). It does a better job of maintaining sharpness (when reducing an image) than the other methods.
- Bicubic (smoother gradients) This method is slower but more precise than the first two (and more desirable). Photoshop spends more time examining surrounding pixels before interpolating new ones. The math at work is very complex, so this method will produce smoother results than Nearest Neighbor or Bilinear.
- Bilinear. This approach uses pixel averaging. It is a balance of speed and quality, and produces medium-quality results.
- Nearest Neighbor (hard edges). This method is very fast but not very precise. It is useful for resizing illustrations. However, it can produce jagged edges.
Setting the Default method
Photoshop allows you to choose a default interpolation method. This will be used when you invoke a sizing command, such as the Free Transform or Image Size command (more on both in the pages ahead). Choose the method that best matches your workflow.
- Choose Edit > Preferences or press Command+K (Ctrl+K) to call up the Preferences dialog box.
- From the Image Interpolation menu, choose your default method (Automatic is the most flexible method and is highly recommended).
- Click OK to store the setting.
Resizing an image
Most of your images will not be sized to the exact dimensions you need. You have several options at your disposal. To change the size of an image, you can use the Image Size or Canvas Size command. You can also use the Crop tool or Free Transform command to make an adjustment. You can use these choices individually or in combination to achieve the desired results.
The Image Size command lets you permanently reassign the total pixel count, as well as resolution, for a particular image. You can also use this command to upsample or downsample an image. This is an easy way to size an image to a specific height or width. Let’s put the command into action:
1. Open the file Resize.tif.
2. Choose Image > Image Size or press Command+Option+I (Ctrl+Alt+I).
The Image Size dialog box offers several choices. You can choose to manipulate the pixel dimensions of the image (measured in pixels or percent). You can also modify the print size, which is the size of the image when printed. You can modify the print size based on percent, inches, centimeters, millimeters, points, picas, or columns. The most common choices are percent, inches, or centimeters, because most users easily understand these units of measure.
3. Set the Document Size to measure in inches. Specify a new height of 6 inches.
4. Be sure to select the Resample Image option if you want to change the pixel dimensions. Choose the method to Resample Image that is most appropriate for your image. Bicubic Automatic is the most common method, but you may have special circumstances. See “Choose an Interpolation Method” earlier in this chapter.
TIP: Return of focus
You can avoid the need for upsampling by scanning or shooting the image at a sufficiently high resolution. If you want to preview the effects of changing pixel dimensions onscreen or to print proofs at different resolutions, resample a duplicate of your image.
5. Leave the Constrain Proportions check box selected, or you will introduce distortion. You generally want to keep the width and height constrained to the same ratio so the image resembles its original appearance.
6. Enter a resolution of 300 pixels per inch for professional printing.
7. Click OK.