[Michael Fournier]

Since I have been doing illustrations, pre-press work for about 12 years now, I thought I might add some to this discussion. Print resolution vs. image resolution is not one-to-one. Pixels per inch needed in a digital image are not equal to dots per inch in the output device, be it an image setter or an ink jet printer or a continuous tone printer.

I will start with continuous tone printers. In these, there are no dots, so the correlation between image resolution and crispness is very relevant. This includes Film setters, Dye sublimation printers and Iris Printers. For this type of printer, an image of around 1200ppi at print size would be recommended for highest quality.

Then follows Photo Ink Jet printers, which today come very close to continuous tone; they still use dots, even though they use a stochastic screen of very small dots of varied sizes. For this type of printer, an image of 2/3 the output device's DPI at full print size is sufficient. In other words if your printer's resolution is 1200dpi and you are printing at 8" x 10", anything higher then 8" x 10" at 800ppi image would be wasted image resolution and would just slow print time. And anything less would start to show in loss of clarity at that print resolution. But at first, the loss would be almost undetectable at normal viewing distance. Remember, the larger the print, the less resolution needed to carry the detail at the intended viewing distance.

So if you wanted to print a poster size image, you may even get away with a 20" x 30" image at 90ppi, without much less in detail noticeable at the distance you would view a poster.

Regarding a billboard size: would you believe these images are printed on large roll-feed inkjet printers at only 60dpi which would give you a 2/3 resolution file of only 40ppi? That is much lower than the resolution of your monitor. But since they are viewed from 100 feet or more away, it does not matter. But that is still one big image, even at 40ppi, if you consider it's about 240 inches wide 120 inches high. Where do I get these numbers? Years of experience.

The last output device is halftone printing devices, like those used to produce lithographic prints. The rule of thumb here is you need 1.5-2 x the lines per inch of your halftone screen at print size, regardless of the DPI of the output device. (Resolution of the output device here is used to get the cleanest halftone dot, not the sharpest image.)

If your image will be printed in a magazine with a 150 line screen at 4" x 5" inches then you would need a 4" x 5" digital image of at least 225ppi; 300ppi would be a good round number. If you were printing a higher quality brochure using a 175 or 200lpi screen (some expensive art magazines use a 200lpi screen) you would need at least a 4x5 at 300ppi - or better yet, 400ppi. If the image were to be printed full page you would need an image that size, say 8.5" x 11" plus bleed at 400ppi. And if the local newspaper is doing a story on you and they want to run an image in the Sunday art section, they would be using an 80lpi screen because newsprint cannot hold a dot finer than that without the darks filling in. So if the image was, say, to be printed 6 inches wide, you would need a digital image of 120-150ppi at 6 inches wide.

The reason a lot of people get mixed up is that dpi (dots per inch) and ppi (pixels per inch) are used to mean the same thing a lot. Photoshop even uses DPI when referring to image resolution. But that is the skinny on image resolution vs. output device resolution.

Of course it never hurts to have too much resolution, except it can take longer to process, and takes up more disk space. But if the resolution needed is not there to begin with or is lost, there is not much you can do to get it back. So it is best to start with the highest resolution file you can get. That way you have an image that can be used for just about any reproduction you may want now or in the future.
Just don't send a 300MB 240" x 120" image to the local newspaper for that article they are writing on you. They won't appreciate it much.