The best resolution to scan your images depends on how it will be used - on screen or in print. For the purposes of this article, we'll assume the image is for print. To find your best scanning resolution you need to calculate the final image resolution, that is, the resolution of your image when placed in your file for printing. Your scanning resolution and final image resolution may be the same, but not necessarily. For most users, it is rare that you will need thousands or even more than 200-400 pixels of resolution.
To reach your final image resolution, you need to know 5 things before scanning:
Size of the original image.
Are you scanning a 4x5 inch photograph, for example? Size of the printed image.
Will your image be reduced or enlarged? LPI at which your image will be printed.
LPI depends on the printer you are using. The type of paper, also affects LPI. Your final image resolution is usually about double the LPI at which the image will be printed. Resolution Multiplier.
This number is usually 2. You can see how it is used below. For most purposes, 2 is the magic number. Editing of the scanned image.
It is often better to scan at a somewhat higher resolution than you'll eventually need to allow for pixels lost during image editing. Just remember, the higher the resolution, the larger the file size. Don't go overboard.
For commercial printing, if you don't know the required LPI, talk to your print shop. They can help you come up with the right numbers based on the specifications of your print project. Here's a quick reference list or see the more detailed chart in the Resolution Inch by Inch article on this site:
To calculate your final image resolution:
If the size of your image will not change, multiply LPI by your resolution multiplier. LPI x 2
For example, an image destined for a brochure on uncoated paper using offset printing needs a final resolution of approximately 133 x 2 or 266. If the size of your image will change, multiply LPI by your resolution multiplier and then by the size of your final image as a percentage of the original. LPI x 2 x %
For example, if our image for the brochure, above, is to be enlarged by 25%, then it needs a final resolution of 133 x 2 x 125% or 333. If it is to be reduced by 25%, then it needs a final resolution of 133 x 2 x 75% or 200. (I've rounded up in each case.) Don't know the percentages? Use the scanning calculator, below.
You may be fine scanning at the final resolution arrived at by one of the formulas, above. However, since most scanned images need some type of image editing, scanning at a slightly higher resolution is often advantageous. It is easier to "throw out" unneeded resolution after scanning than it is to restore lost resolution after the fact. Just remember to reduce your image to the final resolution before placing it in your document for printing.
Having trouble doing the math? Try this simple scan resolution calculator:
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