File Preparation: About Halftones

About Halftones

What is a halftone?

When we look at a black and white photograph we see tone values ranging from a minimum density (usually white) to a maximum density (usually black).All the gray values inbetween these two extremes graduate smoothly from the lightest to darkest.This is called continuous tone because there is no discernible step from one gray shade to the next – they blend smoothly.

A printing press is not capable of printing continuous tones. It either prints solid ink or it prints nothing. To print black & white photos we can only use black ink and white paper. So how do we get all the shades of gray? By creating a halftone from the continuous-tone photo. A photographic image is converted to a series of dots of varying size either by shooting it on a process camera through a halftone screen or more often by scanning it into a computer and printing it to a laser printer or imagesetter.

Light portions of the picture will have very small dots and darker parts will have large dots. These small evenly-spaced black dots on the white paper give the illusion of varying shades of gray. The number of halftone dots per inch is called the screen frequency or screen ruling. An 85 line screen contains 85 dots in each horizontal row and 85 dots in each vertical column for a total of 7,225 dots in each square inch. This measurement of halftone resolution is referred to as lines-per-inch, generally abbreviated as LPI.

At fairly coarse rulings such as 65 LPI the individual halftone dots are easily seen with the naked eye. At a higher ruling such as 150 LPI the individual dots become difficult to see without some kind of magnification. Therefore the higher the screen ruling the more a halftone resembles a true continuous-tone photo. The screen ruling that is used on any particular printed piece is determined by the type of press and paper the job is to run on. Newspapers are generally printed with a 75 to 100 line screen. A sheetfed job printed on a an uncoated stock is generally printed from 150 LPI up to 175LPI. A job printed on a coated sheet can typically be printed at up to 200 line screen on our Komori presses with the Delta Dampening System.

Now that you know what a halftone is, how can you use this knowledge to get the best possible reproduction of your photographic images?

One key to reproducing good photos is to create and use the largest tonal range possible.The tonal range is the range of values available between the lightest and darkest values of a picture.Therefore the greater the difference between the highlight and the shadow of a picture the larger the tonal range of the picture will be. Unfortunately it's not as easy as setting the hightlight value to 0% and the shadow value to 100%. There are other factors you need to account for.

All printing presses create an undesirable printing defect known as dot gain. Dot gain is simply the tendency of a dot of wet ink to spread when it is laid down on paper. The absorbency of the paper and the type of press used are the main factors influencing how much dot gain you can expect to get in a photo. Since uncoated sheets are fairly absorbent as papers go, you tend to get a lot of dot gain when printing on it. One of our biggest challenges when creating a halftone is to anticipate how much dot gain we will get and to compensate for it in our halftone. While our Komori presses with the Delta Dampening System produce less dot gain than a typical sheetfed press, it is a good idea to anticipate a minimum of 15 to 20% mid-tone dot gain on uncoated sheets. This means that a 50% dot on a plate will end up as a 65% to 70% dot when it is printed.

While the term "halftone" is generally used to refer to a single-color image, a duotone can be described as two different-color halftones compsited into a single image, with each color having different tonal values. The same is true for a 4-color/CMYK process color separation.

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