Scanning Tips

If you want to enlarge a photo without loss of detail, use the following formula to determine your correct scanning resolution:

• multiply the print screen frequency by1.5-2.0, then multiply the result by the enlargement factor.
• for example, if you want to enlarge an image 300 percent for a final 150 lpi screen, scan it at 675 to 900 dpi: [(150 lpi x 1.5 or 2) x 3]=a final scan of 675 to 900 dpi.

When you enlarge this 675 to 900 dpi image 300 percent, the resulting larger image will be 225 to 300 dpi: a perfectly acceptable resolution. Remember, enlarging an image reduces its resolution (from 900 to 300 dpi) just as reducing an image increases its resolution.

UPSAMPLING PHOTOS: DON'T DO IT

When you scan a photo for placement in a file that will be offset printed, the general rule of thumb is to choose a resolution 1.5 to 2 times the line screen of the final, printed piece. For example, if you plan to print a photo at 100 percent size using a 150 lpi halftone screen, you would scan the photo at between 225 and 300 dpi. If you intend to shrink the photo to 50 percent of size before placing it in your design file, you can cut the initial scanning resolution in half to (rounding) 113 to 150 dpi. I'm sure most of you have read this rule in many books, magazine articles, and printing seminar handouts.

But what if you want to increase the size of the photo? Simple--you don't. Here's why. When you scan a photo, the computer captures the image in a grid of squares of a certain size (dots or pixels per inch). These squares (together called a raster grid) can be made larger or smaller by enlarging or reducing the photo, but you have the same number of squares after you have enlarged or reduced the photo. If they are reduced, they become less evident to the naked eye, but if they are enlarged, they become visible and soft or fuzzy, or even jagged. The photo loses detail. This effect is called pixellation, and unless it is what you intend, don't scan a photo and then enlarge it. Simple as that.

Of course, all rules are meant to be broken, and if you want a futuristic, computerized effect that draws attention to itself as a product of the computer, fine. If you are looking for a non-realistic, painterly effect, you can also "upsample" (type a larger number in the resolution box, under Image > Image Size in Photoshop), but when you do this, the computer just increases the number of pixels of color in the area (called interpolating) without adding detail.

Photoshop TIFF files are based on rasters, while Illustrator or Freehand EPS files are based on vectors (and are scalable without loss of detail). For the most realistic, least pixellated effect, remember that rasters are like a checkerboard, while vectors are based on mathematically defined arcs and curves. Reduce or enlarge vectors as you will, but avoid enlarging or upsampling rasters.