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If you're not familiar with the various ways for defining resolution, visit my glossary entry for resolution and spend some time studying the related resources there. For this course, we'll only be discussing images in terms of pixels per inch, or ppi. First we need to understand some fundamentals about resolution. Contrary to what you might think, ppi is not the same as dpi. Almost everyone these days has a inkjet printer so let's think about your inkjet printer for a moment. How many colors of ink are in your inkjet printer? Most of them have at least four and possibly six ink colors. When your printer lays down ink, it lays it down in overlapping dots of these four or six colors. Images are made up of pixels and Photoshop defines resolution in pixels per inch. In a typical RGB image, a single pixel represents one color out of the millions of colors that can be reproduced on-screen. So how can a printer with only six colors of ink reproduce millions of colors? Well, certainly not by printing a single dot for each pixel in your image! In reality, for each pixel in an image, your printer may lay down several dots of overlapping color that mix together to make up the color of that single pixel. All this is said to help you understand why you would use an image resolution setting of 150 ppi in Photoshop in order to output an image with a printed resolution of 300 dpi. It's simple; your printer needs to use more than one dot to represent the color of a single pixel. To clear up another confusing matter, resolution (ppi or dpi) has no bearing at all for images that will be viewed on screen. The only thing that means anything for screen display is the pixel dimensions; that is, the number of pixels that make up the width and the number of pixels that make up the height of the image. If you've been told that you need to use 72 or 96 ppi for the Web, it's only because that is approximately what a typical monitor can display. But, because we have the ability to adjust the pixel dimensions of our monitor, and because monitors come in varying sizes, an image with the same pixel dimensions is not always going to appear the same size on screen on every system and with every monitor. However, an image that is 100 by 200 pixels will always be 100 x 200 pixels, unless you resample that image. And that brings us right back to the image size dialog box... Next > Image Size Dialog Box |
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