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Display Onscreen

When you view an image onscreen in Photoshop, you don't see how large your image will be when it's printed. In fact, the resolution setting attached to the image is not taken into account. That's because the program displaying your image (whether Photoshop or a Web browser) does not control the size of the pixels that make up your screen—your operating system decides that. The setting in your Monitors or Display control panel (which is part of your operating system) determines how many pixels you see onscreen, and therefore their size.

Check it out for yourself: On a Mac, choose Apple Menu > Control Panels > Monitors; in Windows, choose Start > Settings > Control Panel > Display. This setting is known as monitor resolution because it determines the size of the pixels that make up your screen. Let's say your image has a resolution of 300 pixels per inch. In order to display an image using 300 pixels in each inch, a 15-inch monitor would have to be capable of displaying at least 3600 by 2700 pixels! (I doubt your monitor can go that high. In fact, most 15-inch monitors display only 1024 by 768 pixels.) When an image is displayed on a computer monitor, the width and height of the image in pixels determines how large the image will appear to be on screen. Photoshop doesn't care whether the resolution of your image is 300 or 72, because it can't change the size of the pixels that make up your screen.

But what happens when you view your image at 200% or 50% in Photoshop? When you zoom in to 200% view, Photoshop can't make the pixels 200% of their current size. Instead, it uses a 2-by-2 pixel area of your screen to show one pixel of your image. That way the image looks twice as big, even though you really haven't changed the size of the screen pixels. Zoom out to 50%, and Photoshop does the opposite—it averages a 2-by-2 area of your image to determine the color of a single screen pixel (this makes the image look half size when you haven't changed the size of the pixels that make up your screen). That also means you aren't seeing all the information in your file any time you view it at less than 100%.

Since the scanning resolution determines how many pixels you'll get out of every inch of the original, then the higher the scanning resolution, the more pixels you'll have in the width of your image. If you scan with a resolution setting that's too high, you might end up with so many pixels that the image will overflow your screen. In that case, it's not the resolution setting attached to the file that matters (the one that shows up in the Image Size dialog box), but the width and height in pixels.

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