Printing & Design Tips: March 2003, Issue #20

Unsampling 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.

Real vs. Fake Duotones

Duotones come in two varieties: "real" and "fake." A real duotone uses two separate negatives and two inks to add dimension to a photo. Each halftone focuses on a limited range from the lightest light to the darkest dark it can reproduce. This equals roughly four f-stops on a camera. Therefore, with one halftone you can emphasize highlights and midtones, or midtones and shadows, but usually not both. Using two negatives printed over one another in two different inks and with their screen rulings angled slightly to avoid moire patterns, one can double the "dynamic range" of the photo. Whether you use two distinct colors, like a black enhanced by a green or a red to give the photo character, or you use two blacks (or a black and a grey) to produce a more naturalistic photo, you can achieve a superior range from highlights to midtones to shadows.

As a side note, a four-color photo is actually a quadtone (as opposed to a duotone). Technically, it is created in much the same way as a duotone: four halftones are laid over one another and set at angles to each other to avoid moire. The only difference in four-color process work is the choice of inks: CMYK inks to approximate full color vs. blacks or browns or variants of gray used in most duotones.

On the other hand, a fake duotone is one halftone of a photo printed in one color over a flat screen of another color. For instance, you can print a black halftone over a twenty-percent screen of blue and get an interesting effect. But it isn't a real duotone.

The best way to tell which is which is to look for the white of the paper (or whatever color the base paper is). A real duotone will have areas with no halftone dots, revealing the paper itself. A fake duotone will show the base screen color (the twenty-percent screen of the second color) through areas of the overprinting halftone that otherwise would be white (or paper colored).

Neither approach is right or wrong; both can be useful options depending on your aesthetic goals for your printed piece. Do keep in mind, however, that process inks and many PMS inks are transparent. Therefore, if you print a dark blue over a screen of gold or yellow, your blue will become green.

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]