Scanning a high-quality photograph takes some thought, especially if you want to scan a small image and make it larger. Making a photograph larger can degrade the quality of the photograph if you scan it at low resolution, resulting in a broken image at larger sizes, an image that lacks colour depth or a distorted image. Using the resolution setting on a scanner in conjunction with image scaling in an image editor allows you to enlarge small images while maintaining the integrity of the photograph.
Set your scanner settings at 300 pixels per inch to scan a photograph at its regular size. Use the ppi setting as the basis for scanning photos larger than original size. Press the scan settings button on your scanner or in your scanning software to select this value.
Set your scan settings to twice the ppi value to double an image. This is the rule of thumb you can use to determine ppi settings for scanning small images to larger. If you have a 2.5-by-2.5-inch image you want to end up as 5 by 5 inches, double your ppi value from 300 ppi to 600 ppi. To size the same image one and a half times larger, use a scan setting of 450 ppi. The more pixels there are per inch, the clearer your image becomes. Too few pixels will leave your image thin and spotty. Since increasing ppi increases the size of your file, don't set ppi higher than you need to for your image.
Open any scanned images that weren't scanned at the appropriate values in a graphics editing program and go to the "Image" or "Tools" menu. Click the "Scale Image" option and set your ppi values and height and width values in the image scale dialogue box. Set the ppi value as described in Step 2, doubling the ppi value for every doubling in image size. For instance, set ppi to 600 if you want to double an image in size and maintain the quality of a 300-ppi scan. Once you've set ppi, click the arrow next to height and raise the value from its current size to the doubled size you want. The width will adjust automatically. If it doesn't, check the "Maintain Aspect Ratio" option so the height and width adjust together. Changing an image's height or width without adjusting the other at the same ratio can distort the image. "Maintain Aspect Ratio" fixes this.