Changing Image Size and Resolution
Pixel dimensions, image dimensions, and resolution are all adjusted using the Image Size dialog box. Since you will often capture one image and then use it for different purposes, it's important to understand how these adjustments affect your image file.
For the Web and other on-screen viewing, it's common to adjust the pixel dimensions, or number of pixels, to control the resolution and/or file size of the image. This is known as resampling. The Resample Image check box is probably the most important feature to image is resampled (Figure 2.19). When the box is not checked, the pixel dimensions are locked in, and no resampling can occuryou can change the document size (the size the image will print) but the size the image displays onscreen will stay the same.
Figure 2.19 Here, an image was duplicated (top) then reduced 70% (bottom). Notice in the zoom views, that understand. When this box is checked, the the reduced image isn't as detailed as the original. pixel dimensions changethat is, the pixels That's because though both have the same number will increase or decrease in number as the of pixels per inch, the reduced image contains fewer pixels overall.
There are no absolute rules for the best resolution to use when scanning your images for the Web or for printing. The best approach is to try a couple of settings, using the following guidelines, and see what works well for your specific situation. Following are some typical situations and recommended resolution ranges:
For on-screen viewing of Web images, 72 ppi is a standard and safe resolution.
For color images printed on color ink-jet printers, a range of up to 150 ppi is often ideal. The exact resolution will depend on your printer and the type of paper on which you are printing.
For color images to be used in printed materials, like a brochure or newsletter, you'll want a higher resolution, usually between 150 and 300 ppi.
If you want to create higher-quality professional projects, such as magazine or print design work, be aware that Photoshop Elements is not capable of producing CMYK files (the color-separated files used for high-end printing). If you need an image-editing program that can handle these kinds of jobs, you should consider buying the full version of Adobe Photoshop.
To resize an image for onscreen viewing:
From the Image menu, choose Resize > Image Size to bring up the Image Size dialog box.
Make sure the Resample Image box is checked (Figure 2.20) and choose an option from the Resample Image drop-down menu.
Figure 2.20 The Resample Image box includes three options for specifying how the resampling occurs.
When you resample an image, its pixels are transformed using a process known as an interpolation. Interpolation is a computer calculation used to estimate unknown values based on existing known valuesin this case, pixel color values. So, when you resample an image in Photoshop Elements, its existing pixels are changed using one of three interpolation methods (Figure 2.21): Bicubic is the default option, and generally produces the best results and smoothest gradations.
Figure 2.21 You can resample an image using one of three calculation methods: Bicubic (left), Bilinear (center), or Nearest Neighbor (right). Bicubic does the best job at retaining detail, and anti-aliasing, while Nearest Neighbor creates images with a rougher quality.
Bilinear produces medium-quality results.
Nearest-Neighbor is the fastest method, but may produce jagged effects. Unless you're resampling a large number of files, then the resampling speed in not a major issue, so we recommend using the Bicubic option in almost all situations.
To maintain the current width-to-height ratio, make sure Constrain Proportions is checked.
Enter new values in the Pixel Dimensions fields. You can enter values in pixels or as a percentage (Figure 2.22).
Figure 2.22 Pixel dimensions can be entered as pixels or as a percentage.
If you choose percentage, then you can enter a percentage amount (1-100) in either the Height or Width box to automatically scale the image to that percentage. The new file size for the image is displayed at the top of the dialog box (along with the old file size in parentheses).
Click OK to complete the change. The image is resized larger or smaller, depending on the pixel dimensions or percentage you entered (Figure 2.23).
Figure 2.23 If you want to reduce an image's file size and display size, you can resize it by reducing its pixel dimensions, making it smaller for posting on the Web or e-mailing to friends.
When you change an image's size onscreen (by changing its pixel dimensions), you also change its print size, as indicated by the changed width and height dimensions in the Document Size fields of the Resize Image dialog box. While these images are acceptable as quick test prints, you may be disappointed with their printed quality. That's because you're discarding image information by resampling (see "Downsampling vs. Upsampling" sidebar, below) and in so doing, lose some sharpness and detail. If you're resizing an image mainly for print purposes, see "To resize an image for print" later in this chapter.
Downsampling vs. Upsampling
In Photoshop Elements, changing the resolution and/or size of an image by adjusting the number of its pixels is known as resampling.
Downsampling, which is the term for decreasing resolution by removing pixels from your photo, is one of the easiest and most common ways to make your files smaller. If you take an 8x10 photograph of your grandmother and shrink it to a 4 x 5 image by reducing its pixel count, you've just downsampled it. Photoshop Elements "throws away" unneeded pixels intelligently, with little or no visible impact to the quality of your image.
But upsampling, which is the term for increasing resolution by adding new pixels to your photo, should be avoided whenever possible. If you take a 4x5 photograph and try to enlarge it to 8 x10, then Photoshop Elements has to add pixels to your photograph. Since it has to manufacture those pixels out of thin air, so to speak, they add no real detail to your image. The end result? A distorted, jagged-edge, pixelated photo.
Since downsampling rarely detracts from the quality of your images, you should capture all your original files at the highest resolution possible, whether you're scanning an image or snapping a digital photo.
To resize an image for print:
From the Image menu, choose Resize > Image Size.
To maintain the current width-to-height ratio, select Constrain Proportions.
Uncheck the Resample Image box.
Choose a unit of measure (or choose percent to scale by a percentage) and then enter new values (Figure 2.24) for the width and/or height in the Document Size portion of the dialog box In the Document Size portion of the dialog box, the resolution value will change accordingly. For instance, if you enter width and height values of half the original image size, the resolution value will double, making the image appear clearer and sharper. That's because you're compressing the same number of pixels into a smaller space. So, when scaled at 50%, an image 4 inches wide with a resolution of 150 pixels per inch (ppi) becomes 2 inches wide with a resolution of 300 ppi.
Figure 2.24 Enter new width and height values to change an image's print size.
Click OK to complete the change.
The image's print size will be changed, but since it still contains the same number of pixels, it will appear to be unchanged on your screen. You can, however, view a preview of the final print size onscreen.
From the View menu, choose Print Size. The image will be resized on your screen to approximate its final, printed size (Figure 2.25).
Figure 2.25 An image can be viewed at an approximation of its final print size, even when its resolution differs from the computer's display.
From the View menu, choose Actual Pixels, or press Alt+Ctrl+0/Option+ Command+0 to return the display size on your screen to 100%.
If things get really messed up and you want to return the dialog box to original settings, press Alt (Windows) or Option (Mac OS) to change the Cancel button into a Reset button. Then click Reset.