A Little Clarity, Part 2

I am designing an ad in InDesign with a color photograph. The ad specs given by the client specify a line screen of 150. I’m not sure what they mean by “a line screen of 150.” I usually place photographs at 300 dpi in my print ads. Does this mean that the photograph should be prepared (in Photoshop) at 150 dpi instead? Do I need to do anything special in InDesign for the rest of the ad (the text)?

All that passage is from a blog I read a few years ago, and the entry was from a person whose job it is to prepare art for print, yet he/she doesn’t know digital resolution from print line screens. And he/she mentions “300 dpi”.

In the first place, “dpi” is a printing term, meaning dots per inch, which sometimes is substituted with “lpi”, meaning lines per inch. They mean the same thing. You may be wondering why those two terms are interchangeable. DPI usage came about because we see dots in the line screen used (above images, middle). Those dots are arranged in rows, or lines.

In order to understand the printing of images, one has to understand screen tones. Reproducing photographs in commercial printing is done by using line art conversions of each color (it can be done using a “digital” method, but that’s inferior). Because printers use four-color process inks (cyan, magenta, yellow, and black) each of those colors must be separated out from the original continuous tone photo (examples, left) to make the separate four screen tones.

You may have come across the term halftone sometime in your career. “Halftone” refers to the fact that the dots, such as in the one-color screened image, cover only half of all the tonalities in the image. Those dots create a semblance of the tonalities and come close to replicating them when fine line screens are used for print.

Sections of the left-hand images above have been enlarged to show the conversion in line screen dots (middle) and pixels (right).

In looking at the screen tones above, you can easily see that smaller dots reproduce lighter tones while the larger dots reproduce darker tones. Newspapers and magazines still use images like this, but with much finer screens. Depending on the quality of the paper used, those publications will use a line screen of anywhere from 85 LPI to 150 LPI. I could go on about what dot gain is, but I’ll save that for a later discussion.

In the digital world, pixels (picture elements) are single-colored (or toned) squares which reproduce photographs on computer and digital television screens. On Windows computers, the native resolution is 96 ppi, while on Macs the native resolution is 72 ppi. When viewing a 400 pixel-wide image on a Windows screen, the picture is 4.166 inches wide. On a Mac, that same image is 5.555 inches wide.

When viewing photos in newspapers and magazines, you’re looking at dots. When you view photos on a computer or TV, you’re looking at pixels. When you scan a photo or transparency for display on your computer screen, you set the resolution at the scanning stage, thereby digitizing the image, which is the digital conversion process. The image you save has been converted to pixels.

Please do not confuse the two. The term “dpi” is not a computer term. When we’re talking about pixels, the term is “ppi”—meaning pixels per inch.

And by the way, the rule of thumb in preparing a photo for print is 1.5 to 2 times the line screen used. Therefore if the screen will be 150 dpi, then the photo should have a digital resolution of between 225 and 300 ppi.


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