After all this explanation of pixels and resolution, you might be thinking, "Okay, this is all very interesting, but what's my bottom line? What Resolution value should I use?" The answer is frustrating to some and freeing to others: Any darn resolution you like. It's true — there is no right answer, there is no wrong answer. The images in this book vary from 100 ppi for screen shots to 300 ppi for color plates. I've seen low-resolution art that looks great and high-resolution art that looks horrible. As with all things, quality counts for more than quantity. You take the pixels you're dealt and make the best of them.
That said, I'll share a few guidelines, but only if you promise to take them with a grain of salt:
♦ Most experts recommend that you set the Resolution value to somewhere between 150 percent and 200 percent of the screen frequency of the final output device. The screen frequency is the number of halftone dots per linear inch, measured in lpi (short for lines per inch). So ask your commercial printer what screen frequency he uses — generally 120 lpi to 150 lpi — and multiply that times 1.5 or 2.
♦ Want to be more specific? For high-end photographic print work, it's hard to go wrong with a Resolution value of 267 ppi. That's 200 percent of 133 lpi, arguably the most popular screen frequency. When in doubt, most professionals aim for 267 ppi.
♦ If you're printing on a home or small-office printer, the rules change slightly. Different manufacturers recommend different optimum resolutions for their various models, but the average is 250 to 300 ppi. Experiment to see how low you can go, though — sometimes you can get by with fewer pixels than the manufacturer suggests. And don't forget that the quality of the paper you use may be more to blame than a lack of pixels for a lousy print.
♦ What if you don't have enough pixels for 267 ppi? Say that you shoot a digital snapshot that measures 768 x 1024 pixels and you want to print it at 6 x 8 inches. That works out to a relatively scant 128 ppi. Won't that look grainy? Probably. Should you add pixels with Image Size or some other command? No, that typically won't help. You have a finite number of pixels to work with, so you can print the image large and a little grainy, or sharp and small. The choice is yours.
♦ What if you have a photograph or slide and you can scan it at any resolution you want? Flat-bed scanners typically offer two maximum resolutions, a true optical maximum and an interpolated digital enhancement. The lower of the two values is invariably the true optical resolution. Scan at this lower maximum setting. Then use Image ^ Image Size to resample the image down to the desired size and resolution, as explained in the "Resampling and Cropping" section near the end of this chapter.
Orson Welles claimed that he relied on his inexperience when creating Citizen Kane. He didn't know the rules of filmmaking, so they couldn't hamper him. When his assistants and technicians told him, "You can't do that," he ignored them because he didn't know any better.
I feel the same about resolution. Take the pixels you have and try to make them look the best you can. Then print the image at the size you want it to appear. If you focus on the function of your image first and fret about resolution and other technical issues second, you'll produce better art.
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