Wednesday, January 31, 2007
January/February 2004 HelpLine
Lossy Versus Lossless Photo Files
Q) As a subscriber to your magazine, I enjoy the articles in it very much. You had discussed what lossy means when I save a photo that has been reduced in size in a photo program. I'm unable to find that information in a back issue. Could you please tell me how saving to a lossy file affects the saved photo?
Fred R. Kudym
A) When you mention lossy, you also have to consider the opposite—lossless—in order to understand what's happening when you compress any data file.
Lossless compression algorithms remove redundant information from a file and allow you to compress a file without losing any information. In other words, after compression, you're able to retrieve all of your original data. If you've ever downloaded a Windows "zipped" file (one with a .zip extension) or a .bin or .sit on the Mac, you've experienced a file that uses lossless compression. This type of compression is needed for text files like spreadsheets, where losing one zero might affect the bottom line.
Lossy compression, on the other hand, throws some data away. JPEG, for example, is typically a lossy compression scheme. It takes advantage of the limitations of human vision to eliminate "perceived" redundant data. But make no mistake—it's throwing away data that you'll never get back. That's why we recommend not saving your original files in JPEG format.
Obviously, digital cameras do save images in JPEG format in order to maximize storage on media cards, but once you offload them into your computer and start to edit them, save them in a noncompressed format such as TIFF or the native image-processing software's format (.psd in Photoshop). If you were to "resave" the file in JPEG, you'd be throwing away more data.
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