Here’s how to scan photos to get the best resolution from your desktop scanner. First, I’ll explain some of the basics and terminology and then I’ll provide a handy table for reference at the end of this post.
Halftones and Lines per Inch (LPI)
In graphics arts terminology, a photograph from your camera is called a “continuous tone” image. Printing presses require a continuous tone image to be converted into a “halftone.” A halftone is created by placing a “screen” made of thousands of dots on the photo and taking a “picture” of it.
Nowadays, this screening process happens using computer software. The screens vary in density and the resolution is measured in lines per inch or LPI. This term refers to the number of dots that the screen places on the photo for every linear inch. The higher the LPI, the smoother the shades look.
Pixels Per Inch (PPI)
Pixels per inch is a unit of measure for scanned images. For example, one photo scanned at a higher PPI than another will have more pixels and the pixels will be smaller. This combination results in getting a better resolution and therefore a higher quality of photo.
Scanning Resolution for Print Images
The resolution you need to scan your photo depends on the size of the original image, the size you want to reproduce it to, and the output method or device you’re using. Your printed output choices include laser, inkjet, digital and offset printing.
I recommend that you scan your photo so that the number of pixels per inch (PPI) at output size is two times the number of lines per inch (LPI). For example, if you are scanning a photo for an offset printing press, your printer may tell you the photo will be screened at 150 LPI. Based on this information, you will need to scan your photo at 300 PPI. If you scan an image at too low a resolution, it may show pixellation, often referred to as “bitmapping.”
Eye-Popping Tip: Another reason to scan your photo at the final size and not enlarge it is because enlarging it will require a higher PPI in direct relation to the amount of enlargement. It would be considered overkill to scan your photo higher than two times the LPI because the PostScript software can’t use the larger file. In addition, it will take a longer time to download and process.
Scanning Resolution for Website Images
Unlike printed output, websites do not have LPI requirements. Still, I recommend you always scan a photo being used on the Internet at the actual size you intend to display it. To prevent having photos that take a long time to download when accessing your website, I suggest you do not scan your photos at a resolution that’s higher than 72 PPI.
How to Scan Photos from Already Printed Sources
Avoid scanning a photo that has already been printed because it has already been “screened” (converted to halftone). If you do, you won’t like the blotchy results you’ll get.
Refer to the chart below for recommended scanning resolutions.
* Always ask your printer for the specific LPI
** Websites do not have LPI requirements