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Grab a ruler (the real thing) and open any image in Photoshop. Make sure that your Rulers are turned on (View>Rulers), choose View>Print Size, and then View>Show Rulers. Now, hold your trusty ruler up to the screen and I'll bet the ruler onscreen and the one in your hand don't match. If Print Size did what it was designed to do, then the two would match and you'd see an accurate view of what size your image would be when printed.
The two rulers don't match because Photoshop makes an assumption about your computer that hasn't been true for more than a decade! When the Macintosh was first introduced, it had a built-in screen that displayed the number of pixels, which was fixed at 72 pixels in each linear inch (ppi). Photoshop assumes that bit of information is still true, even though you'd be hard pressed to find a machine today (Mac or Windows) with a resolution of 72 ppi. Let's figure out the true resolution of your display; then we'll show you how to get an accurate print-size view of your images.
Start by choosing File>New. Set the Width to 1 inch and the resolution to 72 ppi (the height and other settings don't matter in this case), and click OK. Next, choose View>Actual Pixels and if your rulers aren't visible, choose View>Rulers. To determine the actual resolution of your display, hold a physical ruler up to your screen and adjust the percentage setting shown in the lower left of the document window until the onscreen ruler matches the real one. To adjust the percentage, just click on it, type in a new value, and then press Shift-Return (PC: Shift-Enter). Shift-Return/Enter keeps the percentage highlighted so that you can quickly change the setting multiple times.
Once the two rulers match, grab a calculator and multiply the new percentage by 72. In our example, it's 173%, so that would be 1.73x72= 124.56. The number you get is the precise resolution of your specific screen in pixels/inch (every screen will have a slightly different resolution, even if the model and brand are identical). To check it out, create a new document that's 1 " wide and has the same resolution as your screen (124.56 ppi in our example). Next, choose View>Actual Pixels and measure it onscreen with a physical ruler. If it's not 1" wide, then calculate your screen resolution again.
Now let's use Photoshop to display all of our documents correctly in Print Size view. Choose Photoshop>Preferences>Units & Rulers, set the Screen Resolution setting to what we calculated above (124.56 in our case), and then click OK. Now, open any image and choose View>Print Size to see if it's accurate. Compare the onscreen ruler to your physical ruler and...wait a minute, it hasn't improved.
But, with that same document open, click on the Zoom tool in the Toolbox and click on the Print Size button that appears in the Options Bar. That should cause your image to be displayed correctly!
That's right, there are actually two Print Size commands in Photoshop. They should work the same, but they don't: The one in the View menu always assumes your screen has a resolution of 72 ppi, while the Zoom tool's Option Bar uses the Screen Resolution setting in Preferences to determine the true resolution of your screen—therefore displaying images accurately. So, once you have everything set up properly, just ignore the View>Print Size command and instead, always use the Zoom Print Size button.
Ben Willmore, founder of Digital Mastery, a Colorado-based training and consulting company, is a member of the Photoshop Hall of Fame and tours the country with his seminar, "Photoshop for Photographers" He's also author of Photoshop CS Studio Techniques and co-author of How to Wow: Photoshop for Photography.
*Source: Photoshop User (How-To Magazine), January/February 2005
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