Digital images are made up of pixels (picture elements), which can be defined as colored squares. Each pixel is only one color.
A good way to learn about concepts related to image size, resolution, and print size is to experiment with an image using the Image Size dialog box. To do this, open an image in Photoshop. Then choose Image > Image Size, and the Image Size dialog box appears.
Under Pixel Dimensions, Width and Height refer to the number of pixels in an image, which has to do with the size of display on screen. Under Document Size, Width and Height refer to the size of the image when printed.
For Online Display
If you are placing an image on the Web, you may want to resample the image (add or delete pixels) in order to change the display size of the image. To resample, make sure "Resample Image" is checked, as shown above. Unless you want to distort your image, also be sure that "Constrain Proportions" is checked. Then type in a new number of pixels in either the width or height field of Pixel Dimensions.
Sampling up (increasing the number of pixels) makes images larger on a display screen. For sampling up, we suggest that you choose the Bicubic or Bicubic Smoother option from the Resample Image dropdown menu.
Sampling down (decreasing the number of pixels) makes images smaller. For sampling down, we suggest that you choose either Bicubic or Bicubic Sharper from the Resample Image dropdown menu.
Note: The resolution at which an image will display on a monitor is not related to image resolution. Typically the display resolution of a Macintosh is 72 pixels per inch (ppi) and the display resolution of a Windows computer is 96 ppi. This is the case regardless of the setting under Resolution in in the Image Size dialog box. You do not need to be concerned with this setting if you are placing the image on the Web; it only applies to printing.
For Printing
For printing purposes, you may not need to resample your image. Sampling down reduces the number of pixels, which degrades the quality of the print (but does have the advantage of lowering file size). Sampling up (adding pixels) is not usually necessary, because to increase the print size a better method is to lower the resolution rather than sampling up. (Sampling up would unnecessarily increase the file size.)
To change the print size of an image without resampling, uncheck the "Resample Image" box. When you turn resampling off, notice that the Pixel Dimensions are no longer editable.
Under Document Size, there is a tradeoff between Width and Height and the Resolution. If you increase the width and height, resolution decreases. If you increase the resolution, width and height decrease.
As a general guideline, a resolution of 72 ppi may be adequate for general, non-professional purposes. Newsletters and newspapers are likely to require higher resolutions, typically in the range of 150 to 200 ppi, and magazines are likely to prefer high resolution images of 250 ppi or more.
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A good way to learn about concepts related to image size, resolution, and print size is to experiment with an image using the Image Size dialog box. To do this, open an image in Photoshop. Then choose Image > Image Size, and the Image Size dialog box appears.
Under Pixel Dimensions, Width and Height refer to the number of pixels in an image, which has to do with the size of display on screen. Under Document Size, Width and Height refer to the size of the image when printed.
For Online Display
If you are placing an image on the Web, you may want to resample the image (add or delete pixels) in order to change the display size of the image. To resample, make sure "Resample Image" is checked, as shown above. Unless you want to distort your image, also be sure that "Constrain Proportions" is checked. Then type in a new number of pixels in either the width or height field of Pixel Dimensions.
Sampling up (increasing the number of pixels) makes images larger on a display screen. For sampling up, we suggest that you choose the Bicubic or Bicubic Smoother option from the Resample Image dropdown menu.
Sampling down (decreasing the number of pixels) makes images smaller. For sampling down, we suggest that you choose either Bicubic or Bicubic Sharper from the Resample Image dropdown menu.
Note: The resolution at which an image will display on a monitor is not related to image resolution. Typically the display resolution of a Macintosh is 72 pixels per inch (ppi) and the display resolution of a Windows computer is 96 ppi. This is the case regardless of the setting under Resolution in in the Image Size dialog box. You do not need to be concerned with this setting if you are placing the image on the Web; it only applies to printing.
For Printing
For printing purposes, you may not need to resample your image. Sampling down reduces the number of pixels, which degrades the quality of the print (but does have the advantage of lowering file size). Sampling up (adding pixels) is not usually necessary, because to increase the print size a better method is to lower the resolution rather than sampling up. (Sampling up would unnecessarily increase the file size.)
To change the print size of an image without resampling, uncheck the "Resample Image" box. When you turn resampling off, notice that the Pixel Dimensions are no longer editable.
Under Document Size, there is a tradeoff between Width and Height and the Resolution. If you increase the width and height, resolution decreases. If you increase the resolution, width and height decrease.
As a general guideline, a resolution of 72 ppi may be adequate for general, non-professional purposes. Newsletters and newspapers are likely to require higher resolutions, typically in the range of 150 to 200 ppi, and magazines are likely to prefer high resolution images of 250 ppi or more.
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