When you view an image at less than full size, iPhoto smooths it (a technique known as anti-aliasing) to get rid of artifacts that result from scaling down the photo. The relationship between anti-aliasing and sharpening is a trade-off; although anti-aliasing does not modify the actual image file, it does counter the effect of sharpening when viewing the image at less than full size.
Because of this, if you sharpen a photo, close it, then reopen it again at a less than 100-percent view, you may get the impression that your sharpness changes were not saved. In reality they were, but since the image has been re-aliased to fit your screen, the image's true sharpness may not be apparent.
To evaluate whether an image is sharp or needs sharpening, you should view the photo at 100 percent/full size while editing in a separate window. To open the photo in a separate window, you can either Control-click the image and choose "Edit in a separate window" from the shortcut menu, or change your iPhoto preferences:
Now photos will open in a separate window when you double-click them. Once in the separate window, choose 100% from the Size pop-up menu. If you don't see the Size menu, widen the window until it appears.
Tip: If you are editing the photo from one of the other view options, you can press the 1 key to quickly jump to 100 percent view.
Other applications, such as Adobe Photoshop, handle anti-aliasing differently, so the degree of apparent sharpness can vary from one application to the next when viewing an image at a scaled-down size. You should see consistency when comparing the image at 100 percent across other applications.
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