Choosing a Scanning Resolution

Digital images are composed of square pixels, and pixel size is a major factor affecting image quality. The resolution of an image is expressed as the number of pixels per (linear) inch (ppi), or pixels per centimeter. Smaller pixels result in higher image resolution, which generally indicates better image quality. With scanned images, the resolution is also a measure of how much information has been captured from the original artwork.

At relatively low resolution, such as 75 ppi, lines, edges, and character shapes in an image can appear jagged.

When you scan your photos with another application, you should keep the following questions in mind to help you decide on an appropriate resolution:

• Are you scanning line art or text?: Line art, such as pen and ink drawings, and high-contrast images with sharp edges or type, should be scanned at the highest resolution possible. Text scans that will be processed with character-recognition software should also be scanned at high resolution for accurate translation.
• What halftone screen frequency will be used for printing?: For continuous-tone images (photographs), a common rule of thumb is to scan at a resolution of 1.5 or at most 2 times the screen frequency. For example, for offset printing on newsprint at a screen frequency of 85 lines per inch (lpi), an image should be scanned at 128 to 170 ppi. For images printed at 133 lpi, scanning resolution should be 200 to 266 ppi.

Because resizing tends to blur an image, you can use the Unsharp Mask filter to sharpen it. For more information, see Sharpen Filters.

Talk with service bureaus and commercial printers about the screen frequency used for your projects. With this information, you can let Canvas X Draw calculate the optimal resolution for an image. (Resampling and Sharpening Images.)

• Will the final image be smaller or larger than the original?: If you need to enlarge the image, you should scan it at a higher resolution to retain the most information when you resize it. If you will reduce the size of the image, you can scan it at a lower resolution.

About Digital Images and Resolution

Digital images, also known as raster and paint images, are composed of tiny square pixels.

The number of pixels that fit in a linear inch or centimeter is the image resolution. The resolution indicates how much information is in the image, independent of the resolution used to display the image on screen or to print it.

Low-resolution images have larger pixels and look more jagged than high-resolution images. However, while high-resolution images look smoother, they also require more memory and disk space.

See also:

• Sharpen Filters
• Changing Resolution