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Stochastic screening and image resolution: Is there a rule of thumb for conversion?

Answered by Stephen Beals, Digital Pre Press Manager and Writer

In a recent article titled “Is there a rule of thumb about image resolution for printed pieces?” ( the article stated:

Be sure that scanned images (as well as those from digital cameras or CD-ROMs) have a dot resolution (dpi) approximately twice that of the line screen resolution (lpi) used for printing.

While this works fairly well for traditional line screens, is there a similar conversion formula for stochastic screening? I know never to use an image less than 300 dpi at 100% of the printed size, but with stochastic screens, is there the possibility of image degradation if the image file is not higher than 300 dpi?

B. S.

The rule of thumb used for conventional screening also applies to stochastic. But the traditional 300 dpi (dots per inch) resolution at 100% is technically for old printing technologies, where 150 line screens were about as fine as you got. The fact is, today many quality printers are regularly using 175 and 200 line screens or even higher, so 400 dpi is a better choice. Most stochastic print jobs are geared for high quality output.

It might make sense that designers should play it safe and send everything at 600 dpi, but that is really overkill. Even with today's faster processors that chew through data in a hurry, there's really no reason to overdo the resolution. The truth is you would be hard pressed to tell the difference between a 600 dpi file printed with a 200 line screen and a 400 dpi file printed with the same screen. At a certain point, it simply doesn't matter: the human eye can only see so much detail. And the file sizes increase exponentially.

If you have been regularly sending 300 dpi files to your printers even though they are using 200 line screens, there's no real reason to change for traditional jobs. I would advise bumping up to 400 dpi for stochastic simply because the screening techniques vary from system to system. Many printers use hybrid screening: a term which is best understood as a combination of traditional AM screening and stochastic (FM) screening. In any case you will almost certainly be using the equivalent of one of the higher resolution screens.

I'm glad that you pointed out that the 300 dpi rule of thumb applies to images at 100%. Designers and printers need to be concerned mostly with the "effective resolution" of the image. If you have a 300 dpi image, but you are enlarging it 300%, the effective resolution is 100 dpi. This much loss of resolution will be noticeable. Conversely, we have received 200 mb files (so large that could be used on a billboard) to print an image about half an inch square! This not only taxes the RIP, but it also can substantially degrade the image. In extreme cases, too high a resolution can be almost as bad as too low.

But those 72 dpi files that people want printed at 300% will look pretty horrible no matter what screening method is used.

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