What scanning resolution should I use when digitizing my materials?

Your decision about the proper resolution for a digital image should be based on four factors: (1) the level of detail you wish to preserve, (2) your intended output medium, (3) your desired output size, and (4) any constraints on the file size of your digital image.

Detail preservation
This factor is the most straightforward: the higher your scanning resolution, the more detail you will preserve. A 600-ppi (pixels-per-inch) scan will capture four times as much information per square inch as a 300-ppi scan.

The ideal amount of detail preservation varies among materials. For example, a simple page of standard-size text will not require as detailed an image as an intricate line drawing. A good starting rule of thumb is that a high-quality 300-ppi scan will clearly capture textual characters down to 2 mm wide, while a high-quality 600-ppi scan will clearly capture textual characters down to 1 mm wide. Likewise, a 600-ppi scan will clearly capture detail in a simple piece of line art, while a 1200-ppi scan may be needed to fully capture the detail in a highly intricate piece of line art.

Output medium
A slightly more complicated consideration involves what medium you intend to use for viewing your scanned images. In general, higher-resolution images are required for quality print reproductions than for on-screen viewing.

If you will be using your digital images to create prints, you will want to use an initial scanning resolution at least 50% larger than the line screen (lpi) of your output printer. Thus, if you will be printing at 200 lpi, the resolution of your digital images should be at least 300 ppi.

If your images will be displayed on-screen, you will want to consider the likely display resolution of the images. Currently, the most common computer monitor resolutions are 1024x768, 1280x1024, and 1600x1200 pixels. Knowing your desired output pixel dimensions will allow you to determine how many pixels per inch to use when scanning. For example, a 8?x6? document will need to be scanned at a resolution of 200 ppi to produce an image that will fill a 1600x1200 pixel screen.

If your images may be used for multiple purposes, you should produce original scans at the highest resolution necessary and then make derivative copies of the images for your lower-resolution purposes. When scanning images for archival purposes, it is useful to remember that output capacities that seem high today may become standard in the future.

Output size
A third factor to consider is the intended output size of your desired images. You should increase your scanning resolution if your output images will be magnified when compared to the original images. For example, if you want to produce 8?x10? prints from 4?x5? originals, you should plan to use twice the scanning resolution that you would use if you were making 4?x5? prints. Similarly, to sufficiently magnify a 35-mm (1??) slide to fill a 1600-pixel screen, you will need to capture around 1200 pixels per inch. If you wished to be able to zoom in on this image so that a quarter of the image could completely fill the screen without losing detail clarity, you would need to capture at least 2400 pixels per inch.

File size
The final factor to consider is that any increase in scanning resolution will significantly increase the size of a digital image file. For example, doubling the resolution will produce a four-fold increase in file size. Thus, your decisions regarding image resolution should always be constrained by the limits of your available storage space and your desire for efficient file transfer times.

by Julianne Smith