Since the release of iPhone 4 and the iPad 3, Apple has doubled the resolution of the displays, which are now 640 x 960 pixels (iPhone 4 and 4s), 1536 x 2048 pixels (iPad 3), and 640 x 1136 pixels (iPhone 5). To keep a good-looking user interface for both the old as well as the Retina resolution, Apple decided not to resize all graphics or make use of scalable image formats (such as SVG), but instead it now requires two sets of graphics for each device. When building an app for iOS, you have to provide the normal-sized and double-sized images for each graphic. This is where the strongest Adobe Fireworks feature comes in.
When creating your new document, choose the @2x-resolution (2x) as your canvas size (for example, this means 640 x 960 px for iPhone 4/4s in portrait orientation) and keep the PPI at the default of 72. iOS ignores the PPI setting stored in PNG files — the only thing you need to care about is to double the resolution (in pixels) of our images when working on Retina display designs. Starting with the higher resolution is Ivo Mynttinen preferred way to design apps for Apple devices, as it allows him to set the appearance of each single pixel, and add details which won‘t be visible on the scaled-down version. In the worst case, if you have to use bitmap textures for some reason, the scaled-down version will look a lot better than a scaled-up one (which will have blurry textures).Read the full article