Photo Scanning Tips

Picking a Resolution Is Important. The best resolution to scan your images depends on how it will be used--on screen or in print. Let's 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.

It helps to know 5 things before scanning:

1. Size of the original image.
   Are you scanning a 4x5 inch photograph, for example?
2. Size of the printed image.
   Will your image be reduced or enlarged?
3. DPI (dots per inch) or LPI (Lines per Inch) at which your printer will print the image. This depends on the printer you are using. The type of paper, also affects LPI. You can see typical lpi in the chart below. 
4. Resolution Multiplier.
   A common rule of thumb is to multiple the LPI at which the image will be printed by 2. For most purposes, 2 is the magic number. (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. So 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 at about.com:

the printer in CL145

240 dpi

screen printing	35-65 lpi
laser printer/photocopier	50-110 lpi
offset printing	60-150+
standard for newsprint	85 dpi
standard for offset printing	133 dpi

The basic rule for resolution when scanning is really very simple. For printers whose resolution are a multiple of 300 dots per inch (dpi) nothing over 300 pixels per inch for the photo will make any difference when you print. So for anything you may print at the same size, set the scan resolution for at least 200 ppi and no higher than 300 ppi. For printers whose resolution is a multiple of 360 dpi, the equivalent scan resolutions are 360 ppi and 240 ppi.

If you think you might crop the image or print it at larger size, scan it at a resolution that will give you at least 240 ppi after it's enlarged. For a 4 by 6 that you might print at 8 by 10, for example, 8 divided by 4 is 2. So if you want 200 ppi when you print at 8 inches, you should scan at 400 ppi.

If you never expect to enlarge the image or even print it, 72 ppi is fine for viewing on screen--or 132 for the iPad; 220-300 ppi for retina displays.