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Improve the quality of printed iPhoto books
That's wrong on many levels. It's quite simple, actually. The "DPI resolution" is essentially an artifact of printing, and has nothing to do with the quality, physical size, or disk size of an image. This is perhaps one of the most confused pieces of computer knowledge I've encountered. I recommend not even speaking of DPI except in the context of printing.
Computer displays have a fixed pixel pitch (around 100 pixels per inch on Apple displays). An image with a given pixel size (say 2048x1536) will be displayed dot-for-dot in most browsers and image manipulation programs (at 100% zoom) on the display, or around 20x15inches (which, except for those of you with giant cinema displays, will require a trip to the scroll bar). That's why full-resolution digital camera images look so large at native zoom (one pixel=one pixel). DPI has no meaning except in the context of printing an image. Why? Printers are different. Given an image of size 2048x1536, they can print that image in a 3x2 inch area, a 6x4 inch area, whatever. At some print sizes, the printer can't make use of that information to increase the print quality -- it is useless additional resolution. E.g., for a 3x2inch print of a 2048x1536 image, you'd need a printer capable of ~700DPI printing to actually gain any quality. The converse is also true. If you want a 300dpi image printed at 4x6inches, you'll need a source image of at least 1200x1800 pixels to meet this. If you use a smaller image, the printed output will be interpolated to fill in the missing information. This often produces acceptable results, but doesn't really enhance the resolution (Star Trek "isolate and enhance" magic image manipulation aside). A reasonable target printing resolution is 300DPI, for any sized image. This looks good from a foot or so away. One important side topic: larger images don't necessarily mean better images: a 1200x1800 pixel image horribly out of focus will of course look far worse printed than a nicely focussed 600x900 image printed at the same size. The image dimensions in pixels just give you a feel for the maximum possible resolution obtainable. Apple switches to 150DPI (which is inferior, and far below what any reasonable print device can handle) with the larger images to save bandwidth and file size on their hard disks, plain and simple. Some programs let you tag an image with a "DPI" header. This doesn't change it in anyway, but is just a suggestion for printing and composition systems down the line. It is actually respected and applied in very few cases. Usually the final application doing the printing makes all those decisions. |
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