1.iPhone5分辨率320x568，像素640x1136，@2x
2.iPhone6分辨率375x667，像素750x1334，@2x
3.iPhone6 Plus分辨率414x736，像素1242x2208，@3x，
不处理时自动等比拉伸，如果在老工程打印屏幕frame，依然是320x568
更多信息参考：
-
Icon and Image Sizes
https://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/IconMatrix.html#//apple_ref/doc/uid/TP40006556-CH27-SW1
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Launch Images：
https://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/LaunchImages.html#//apple_ref/doc/uid/TP40006556-CH22-SW1