Resizing Images
There are three ways to change the size of a digital canvas - changing the canvas size without changing the image size, changing the image size without resampling, and resampling the image. In the first and last case, the aspect ratio (the width of the image divided by the height of the image) may also be changed.The examples below are given for Photoshop Elements, but most software packages will have similar methods. The sample image used on this page is taken from the image at right.
Canvas Size
The first method is to modify only the canvas size without modifying the image size. This will result in a border around the original image (if the new canvas size is larger) or the cropping of the original image (if the new canvas size is smaller). This can be done in Photoshop Elements with "Image > Resize > Canvas Size". When changing the size of the canvas, it is possible to change the width and the height separately, which will change the aspect ratio of the image.Image Size
When changing the image size, there is the option to resample the image or not. Resizing without resampling changes the resolution of an image by changing the size of each pixel (and therefore size of the whole image) without changing the number of pixels. In Photoshop Elements, an image can be resized without resampling by choosing "Image > Resize > Image Size" and unchecking the "Resample Image" box. This option does not allow the aspect ratio to be changed.Resolution is commonly measured in spi (samples per inch, used in scanning), ppi (pixels per inch, used for screen display), dpi (dots per inch, used for printing), and lpi (lines per inch, also used in printing). This web site will use the term dpi to cover all units of resolution.
DPI is calculated by the number of pixels in an inch, so dpi = pixels/inches, which can also be written as inches = pixels/dpi or pixels = dpi x inches. When the image isn't resampled, the relationship pixels/inches = dpi is maintained. Since the number of pixels does not change, changing either inches or dpi forces the other to change.
An image that is 720 x 720 pixels will be 3 x 3 inches at 240 dpi (720 pixels / 240 dpi = 3 inches).
- Changing the size of the image changes the dpi. Changing a inch 240 dpi 720 x 720 pixel image to 4 x 4 inches makes the image have a180 dpi (720 pixels / 4 inches = 180 dpi).
- Changing the dpi changes the size of the image. Changing a 240 dpi 720 x 720 pixel image to 360 dpi makes the image 2 x 2 inches (720 pixels / 360 dpi = 2 inches).
Resampling Images
Resampling changes the number of pixels in an image, which results in a loss of image quality. In Photoshop Elements, an image can be resampled by choosing "Image > Resize > Image Size" and checking the "Resample Image" box. The three options available for resampling are nearest neighbor, bilinear, and bicubic. In general, bicubic is the best choice. When resampling, the aspect ratio remains fixed unless the "Constrain Proportions" box is unchecked. Unchecking this box will allow the width and the height to be changed separately, changing the aspect ratio.- Changing the number of pixels changes the size of the image. Changing a 240 dpi 720 x 720 pixel image to 240 x 240 pixels makes the image 1 x 1 inch (240 pixels / 240 dpi = 1 inch).
- Changing the size of the image changes the number of pixels. Changing a 240 dpi 720 x 720 pixel image to 2 x 2 inches makes the image 480 x 480 pixels (240 dpi x 2 inches = 480 pixels).
- Changing the dpi changes the number of pixels. Changing a 240 dpi 720 x 720 pixel image to 360 dpi makes the image 1080 x 1080 pixels (360 dpi x 3 inches = 1080 pixels).
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