Commercial Printing: Resampling Photoshop Images

I received an email last night from an associate who is a print book designer. The email read, “What is the proper resolution for an image in Photoshop?” I knew he had the answer: Twice the line screen of the printed image (i.e., 300 dpi for a 150-line printer’s halftone screen with the image reproduced full size—no enlargement or reduction).

What my associate really meant, when I got him on the phone and we discussed this further, was that he had a 72 dpi web image, and he needed to use it in a print book. “Don’t do it,” I said. “It’s always, always, always better to rescan the image at the proper resolution.”

Unfortunately, the only copy he had, or could get from his client, was the 72 dpi image.

Why Upsampling (i.e., Interpolation) Is Dangerous

First of all, why shouldn’t you upsample an image? That’s the technical term for enlarging an image in Photoshop while resampling it to increase its resolution. When you make a photo smaller, you squeeze up the image pixel information (i.e., you put the same number of pixels in a smaller space) and actually make the image appear sharper. However, when you enlarge it, you magnify the flaws.

More precisely, if you change the resolution of a 72 dpi image to 300 dpi, you are actually creating image information from scratch (making it up, which is called interpolation). The enlarged and resampled image may well have such flaws as jagged edges on items in the picture and/or a hazy, unfocused appearance. Even going back into the photo and resharpening it won’t make it right.

If you just enlarge the photo without resampling it (i.e., magnify the 72 dpi image without changing the resolution to 300 dpi), you’ll see the pixels as a pattern of huge, unattractive squares.

Just Don’t Do It

So the goal is to avoid interpolation like the plague. I said as much to my associate, but when I realized that he had to use the image and he had no other alternative, I taught him a trick.

Several years ago I had actually been successful in enlarging the cover photo of a print book using this technique, which I found online under the title “The Dark Art of Upsampling.” When I searched the Internet last night, I couldn’t find the exact article, but I did locate a number of other articles under that title.

Here’s the technique:

1. 1. Let’s say you have a 72 dpi image that’s 8” x 10” (576 pixels x 720 pixels) and you need to reproduce it as a 4” x 5” photo in a print book.

1. 1. Fortunately, since you’ll be using it at half the size of the original, you really already have an image that’s double the 72 dpi resolution, or 144 dpi. This is because reducing the size of the image increases its resolution commensurately.

1. 1. So without doing anything other than reducing the size of the image and therefore packing the pixels closer together, you have a 4” x 5” image that will print at 144 dpi. But you need an image that’s about twice that resolution.

1. 1. So, you open the image in Photoshop, and then open the “Image Size” dialog box. Then, instead of using a pixel dimension for the target image size, you change the dialog box units of measurement from “pixels” to “percentage.”

1. 1. You then choose “Bicubic, Smoother” at the bottom of the Image Size dialog box (an option created specifically for enlarging and resampling images).

1. 1. Finally, you choose a target size of between approximately 105 percent and 110 percent. Then you repeatedly enlarge the photo as many times as you need to to bring it to its final size. Apparently, by upsampling the image in small increments of 105 to 110 percent, using this “bicubic, smoother” technique, you can minimize the flaws that would otherwise usually appear. But only enlarge the image in 5 to 10 percent increments at a time. Then, if at all possible, don’t use the photo for an important image like a cover shot on gloss coated paper. Instead, consider it for a large format printed image on a building wrap or bus wrap, where it will be seen from a distance.

1. 1. Check the image very closely (at a high magnification) in Photoshop for any jagged edges in elements of the photo with straight or diagonal lines. Also look for “halos,” “artifacts,” or any other image information or patterning that shouldn’t be there.

1. 1. Then forget this technique, or try not to use it again. It is never as effective as using a properly scanned, crisp image at the correct resolution.

1. If the technique doesn’t work for you for whatever reason, there are dedicated software packages that do essentially the same thing.

Some final words: Just don’t do it.

This entry was posted on Wednesday, June 18th, 2014 at 1:55 pm and is filed under Photoshop. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.