Resolution

Resolution also known as DPI (Dots Per Inch) or PPI (Pixels Per Inch) can be described as the number of dots that fit horizontally and vertically into a one-inch space. Generally, the more dots per inch, the more detail captured and the sharper the resulting image.

Help Topics

For an image to print properly, the image resolution must be at least 300 dots per inch (DPI) at the final output size.

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How to resample image in Photoshop
How to check resolution in Illustrator

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If your file is not 300 dpi, you can not simply increase a low resolution image to a higher one by increasing the DPI in your imaging program. The printed result will be a blurry image.