Scanner Resolution and Color Depth

A deeper understanding of dot per inch and other specs

Epson Perfection V19 Color Scanner
Excellent photo scans on a budget. Epson

Image resolution and color depth are crucial to all types of imaging, not just scanning. Peter's 2008 article on scanning basics is a good overview. 

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For scanners, resolution refers to the amount of information, calculated in dots per inch, that the scanner can read. More dots equals higher the resolution, and thus better-looking scans (low-resolution scans sometimes have big and obvious squares of pixels).

Keep in mind, however, that high-resolution scans are liable to have mighty big file sizes, meaning they’ll take up a lot of room on your hard drive, and could take a while to open, edit, and print.

How Much Resolution Do You Need?

How high a resolution you need depend on how you’re planning to use the image.

  • Web posting: Most computer monitors can display 72 dpi maximum (a little higher for high-definition monitors), so if you’re scanning something that will be seen on the Web or will go in an e-mail, scan at 72 dpi. You won’t lose anything if you scan at a higher resolution, but you won’t gain anything either (except in terms of file size, which could mean a lot to those on the other end of the e-mail).
  • Photos: Unless you’re planning on enlarging photos, you’ll get great picture quality by scanning at 300 dpi. If you’re going to double the size of the original, double the dpi.
  • Image editing: Follow the “double the dpi” rule if you’re planning on cutting the original down by cropping it.

     

    Color / Bit Depth

    Color or bit depth is the amount of information the scanner gets about the thing you’re scanning; the higher the bit depth, the more colors get used and the better looking your scan will be. Grayscale images are 8-bit images, with 256 levels of gray. Color images scanned with a 24-bit scanner will have nearly 17 million colors; 36-bit scanners will give you more than 68 billion colors.

    Again, the trade-off is toward huge file sizes. Unless you’re a professional photographer or a graphic designer, there’s not much need to worry about bit depth, since most scanners have at least 24-bit color depth.