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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A Little Clarity, Part 1

I come from the advertising/print industry originally, though I’ve been in the advertising/digital industry for the past twenty-three years. And I’ve seen some terminology transposed from one to the other. Sometimes that terminology works in the translation while other times it does not.

And so, being the nit-picker that I am, I’m starting a new category on those terms that bug me the most.

The one that bugs me the most is “dpi” in this digital world. But I’m going to save that one until later. Right now I want to talk about this “#” symbol.

It’s been called a “number symbol”, a “pound symbol”, and lately a “hash symbol”. I have news: it’s all of those. But first, let’s not confuse this symbol with a few others that are very similar in appearance, such as the sharp symbol in music, that being ♯, or a Chinese character, that being .

A little history: the Romans had a symbol denoting pound weight, that being “℔”. Over time, this symbol was simplified to look like the featured symbol pictured above. Usage of the symbol for pound weight goes back to at least 1850 in this country. By the second half of the 19th century, there were typewriters in the United States equipped with the # key, and the user’s manual reflected the use of that key being specifically for expressing both number and pound weight, depending on placement of the symbol—just before a number to denote the ordinal number itself, and just after a number to denote pound weight.

Printers and paper salesmen (salespersons in the PC world) have long used the “#” symbol after a number to denote the pound weight of paper in text and cover stock, such as “24# text”. Manufacturers of pencils refer to the ratio of graphite/clay used in the writing instruments by a number designation, such as “#2 pencil”, an example of an ordinal distinction.

This traditional usage prevailed until the advent of digital telephone keypads, where the symbol became known as the pound sign. Today we still refer to that as the pound sign on phone keypads. The font used on your specific phone for that symbol may vary, but the designation is the same.

In the United Kingdom, the symbol has been referred to as a “hash”, a derivative of the word “hatch”, meaning cross-hatch (artists will recognize that reference, those of us who’ve done pen & ink sketches with a cross-hatch technique to add tonality to the basic black and white medium). That likeness is probably the best reference we have as to how the term hashtag came about, and was introduced by Chris Messina, a noted advocate for open Internet standards, in 2007 to denote a metadata tag for group use on social networks such as Twitter (but even he referred to the symbol as “pound” when first trying to establish the tag). You know the symbol is a hash when it precedes a word or phrase.

So it depends on circumstance as to the name of the pictured symbol.

Next week, I’ll dive into that “dpi” thing.

 

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