One thing to master about scanning is the proper scanning resolution. The proper resolution to scan depends on how you intended it to be used.
Before scanning it is important to know these five vital information: size of the original image, size of the printed image, LPI at which your image will be printed, resolution multiplier and the process of editing the scanned image. The hint is – the higher the resolution, the larger the file size.
If the size of the image is absolute, multiply LPI by your resolution multiplier. Ex. LPI x 2 for brochures.
if the size of the image is not absolute, multiply LPI by your resolution multiplier and then the size of the final image as a percentage to the original. Ex. LPI x 2 x %.
In cases of commercial printing, ask your print shop if you do not know the required LPI as well as other specifications.
Scanning at a slightly higher resolution is most of the time advantageous. By that, it is easier to dump unneeded resolution after scanning. Moreover, reduce your image to the final resolution before placing it in your document for printing.
In most of the print works, 300 dpi is the standard print resolution. The standard resolution for web is 72 dpi. This works best in computer screens. If you you’re scanning in an image and only going to use it for web you could maintain it at 72 dpi. Film print’s standard resolution on the other hand is 720 dpi. That way you can make enlargements. The general rule with resolution is that never go up but you can always go down. If you have a 720 dpi image, you can at any rate bring it down to 300 dpi and still look sound.
If you are using photographs for your web, scan them at 72 or 100 dpi.72 dpi is screen resolution and the preferred format for saving the image is jpeg though you can also use .psd. Remember that when you Save As, you are creating a copy of your current document and you are decreasing its file size. You can also save your file within Photoshop. This will allow you to preview your work to adjust the quality of the same. If the image is really huge, simply go to Image > Image Size and bring down the resolution to 300 for print or 72 for web or change the image dimensions if it is indispensable.