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There's No Such Thing as Resolution on the Web!
And Not in Your Digital Camera, Either!
by Pete Bauer

Many of us were raised in the business of Web design under the old rule "Everything must be 72 dpi!" It's ingrained. Burned into the brain. And wrong. Seventy-two dots (or pixels) per inch is not a valid measure for today's monitors. Hey, think about it -- you've got variable resolution, even on most portables now. When you switch among the various screen resolutions, the "size" of objects on the monitor changes, right? So an "inch" at 800 pixels by 600 pixels would be different that an "inch" at 1024 x 768. Which one is the true inch? Well, it so happens that on many 17" monitors set to 800 x 600, an inch is indeed very close to an inch.

Images 1-3 show a scan of a ruler (done at 72 dpi -- scanners DO have resolution), all on the same 17" ViewSonic monitor. In the first, the monitor is set to 1024 x 768 pixels, in the second to 800 x 600, and in the third it's 640 x 480. Observe that the ruler is a different size in each.

When working for the Web, we can't think of a certain image as being "this-many-inches by this-many-inches at 72 ppi." We know that the monitors of our Web page's visitors are set to different resolutions, so the number of inches occupied by an image will vary. Rather, we need to think strictly in term of "this-many-pixels by this-many-pixels." With differing resolutions, the size of the pixels will vary from monitor to monitor, but the number of pixels occupied by the image remains the same. Just as a Web page should be planned in pixel dimensions, so to should the components of that page.

Image #4 shows the display properties of a somewhat-elderly Winbook running Windows 95. The "Desktop Area" can be set for the current 800 x 600 pixels or 640 pixels x 480 pixels. And note that under "Font Size" is shows 96 dpi. (Which, by the way, is also wrong.)

Image #5 is the Monitors Control Panel of Mac OS 9. In this case, the above-mentioned ViewSonic monitor is connected, and its recommended resolutions are visible.

Let's take a look at Photoshop 5.5's Image Size dialog box. The box is divided into two sections: Pixel Dimensions and Print Size. Note that the word "Resolution" appears only under Print Size. The image recorded on your disk and displayed on your monitor has a certain pixel size. Resolution is merely how tightly the printer packs those pixels.

Scanners work backward. They take a physical object with a certain physical size and record it as pixels. You tell the scanner how many pixels you want recorded for each square inch of the object. Once in the computer, the object's image is again a specified number of pixels. If you want to output to a printer at "life size," you simply print at the same resolution as the scanner's capture.

The issue of pixels vs. inches is also hot in the digital camera world. Many new owners of such devices have no prior experience with image editing programs. They are astounded by the "size" of the new photos. One novice recently asked me if he'd made a serious error by purchasing a three-megapixel camera. "My printer doesn't take paper that big!" he said. Indeed, when he opened his best-quality images (9 MB), he was seeing an image that was over 28 inches x 21 inches at 72 dpi.

A more common mistake is resampling. Many experienced Photoshop users are needlessly resampling digital photos to bring them down to a suitable print size. Such image alteration should be avoided when possible. While Photoshop's bicubic resampling does a wonderful job, it still isn't as clean and crisp as using the original pixels. That 28" x 21" image noted above started life with pixel dimensions. At 2048 x 1536 pixels, there's a lot of information with which to work. Rather than resampling to reduce it to a printable size, simply tell the printer to pack the pixels more tightly. Under the menu command Image> Image Size, uncheck Resampling before changing any dimensions. Now, rather than changing the size and resolution to your target output dimensions, simply change the size. Photoshop will automatically recalculate the resolution, cramming all of the existing pixels into the space you've requested. When dropping from 28 x 21 to a more common 5 x 7 inches for output to an inkjet printer, the new resolution is more than adequate for the job. (See Image #7. And don't worry, the print driver will round off the fractional resolution.)

The image requires a bit of cropping to drop the width (which it would require anyway as long as the proportions are constrained), but only the original pixels will be output. By avoiding resampling, we've can retain a photo's original sharpness.

By thinking in terms of pixels instead of inches, we can avoid introducing the blurring that comes with resampling. Today's digital cameras do a pretty good job, and many of them certainly can record a huge number of pixels. Use those original pixels whenever possible.

This article by Pete Bauer was reproduced as originally published by http://www.planetphotoshop.com

Pete Bauer & KW Media Group are in no way affiliated with Midwestern Industries, Inc.

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page last updated: Sunday, 12-Jun-2005 12:00:06 CDT