Q&A: How do pixels relate to the physical dimensions of a cover?

Discussion in 'How to: Q&A' started by ctaulbee, Dec 7, 2005.

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  1. ctaulbee

    ctaulbee Demigod Moderator Staff Member Site Staff

    Pixels is the name given to the dots on the screen or dots of ink from a printer. As all images are made up of pixels which are actually square when viewed in a graphics program like Photoshop.

    The DPI is simply the spacing of the pixels for a printed image or dots per inch or number of ink dots printed per inch, has nothing to do with image quality that is a myth, only total pixel count and image quality setting used when the image is saved determine that.

    So a 3000 X 2000 @ 72dpi is the exact same quality as a 3000 X 2000 @ 300dpi so long as everything else is the same, since your screen can only display 72 dpi anyway graphics programs ignore this setting, they only display pixels, the DPI it is for the printer only. The difference between DPI settings is the final printed image physical size.

    Since the DPI sets the physical size of the image for a given pixel count. A fixed pixel sized image with a 300 DPI setting results in a print out size that will be smaller as the dots will be placed closer together, than for say 150 DPI which would be twice as big as the dots would be farther apart, or 72 DPI bigger still as the dots will be even farther apart. The standard used for most general printing is 300dpi.

    A standard case cover image that is 3240 X 2175 pixels @ 300dpi will equal 10.8in X 7.25in printed.

    Remember when you see pixel sizes here it is assumed that your talking about a 300dpi image. so dividing the pixels by 300 will give you the inch size of the image printed.

    But remember that an image that is 3240 X 2175 pixels can print at different sizes depending on what the DPI is set to. Because if the DPI goes down and the pixel count remains unchanged the printed image will grow larger as it is a ratio and the dots in effect are being spacing out from each other. As the DPI goes up the dots will get closer together so the printed image will shrink if the pixel count remains unchanged.

    So... for a 3240 X 2175 pixel image with the following two DPI settings:

    First if the DPI is set to 150dpi then printed it would printout at 21.6in X 14.5in on the printer.

    Then the same image...

    But with the DPI changed to 300dpi will now printout as 10.8 X 7.25in on the printer.

    Yet both would be the same 3240 X 2175 pixel source image with only the DPI changed.

    So you need to watch the Document Size vs Resolution under the Image Size Tab in PhotoShop.

    Hope this helps.
    Last edited: Jan 1, 2013
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