Image Size and Resolution
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PSDTUTS Photoshop Wiki / Photoshop Tutorials / Image Size and Resolution
Photoshop allows you to modify your image’s size and resolution, but there are some factors to consider before attempting this.
In order to change the size of an image, Photoshop needs to make various decisions as to which pixels it expands (when the resize makes the image bigger) or removes (when the resize makes the image smaller). Because the nature of image resizing involves manipulating pixels in this manner, you can only generally add a third of an image’s existing size to it before it starts to pixelate and look like a low-quality JPEG image—and even less than a third in many cases.
Resolution is the number of pixels in an inch as displayed on your screen (ppi). Higher ppi means a higher resolution, which translates to better quality. When you resize an image, you’ll generally be changing the resolution in tandem. For instance, when you make an image larger, there are fewer pixels in one inch, hence the low-quality look.
With these factors in mind, let’s look at image resizing and resampling.
To Change Image Size
First, head to the Image menu and select the menu option Image Size. A dialogue box will appear. There are two main sections: Pixel Size and Document Size: Note that the size on your screen, or pixel size, is not necessarily representative of the size it will be when it is printed, the document size. The relationship between the two is decided by the Resolution field—the default, though, is 72 dpi. For most screen applications, 72 dpi is optimal and will produce accurate results.
To demonstrate the relationship between resolution, pixel size and document size: if the resolution of your image is 100 pixels per inch, and the image is 2x2 inches in size, then the pixel size will be 200 in height and 200 in width.
Changing image size can hence be done by modifying the pixel height and width, which is best if you’re designing images for a website, or by modifying the document size in inches or centimetres, which is best if you’re designing for print, especially when you know the exact dimensions of the paper being used.
To prevent the height or width of the image scaling with changes made to the other variable, uncheck the Constrain Proportions box.
To Change Image Resolution
To change the resolution, simply enter a new number in the Resolution field and ensure that the correct measurement—inches or centimeters—is selected.
If you want Photoshop to change the resolution without changing the pixel size, ensure that Resample Image is unchecked. This will still modify the print size of your document. When Resample Image is checked, Photoshop will keep the document/print size as constant.
Remember that when working with designs for screen, such as web designs, that you want the resample checkbox ticked, the dpi set at 72, and you should use the pixel dimensions to select your image size.
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