the difference among ios deivces

device name	screen size	resolution	PPI	aspect ratio
iPhone	3.5 in.	320*480	163	3:2
iPhone 3G	3.5 in.	320*480	163	3:2
iPhone 3GS	3.5 in.	320*480	163	3:2
iPhone 4	3.5 in.	640*960	326	3:2
iPhone 4S	3.5 in.	640*960	326	3:2
iPhone 5	4   in.	640*1136	326	16:9
iPad	9.7 in.	768*1024	132	4:3
iPad2	9.7 in.	768*1024	132	4:3
the new Pad	9.7 in.	1536*2048	264	4:3
iPad mini	7.9 in.	768*1024	163	4:3
iPad 4	9.7 in.	1536*2048	264	4:3

 

What’s PPI?

Pixels per inch.

What’s Retina Display ?

Retina Display is a brand name used by Apple for liquid crystal displays which they claim have a high enough pixel density that the human eye is unable to notice pixelation at a typical viewing distance. The term is used for several Apple products, including the iPhone, iPod Touch, iPad, and MacBook Pro.^[1] As the typical viewing distance would be different depending on each device's usage, the pixels per inch claimed as retina quality can be different for the smallest devices (326, iPhone and iPod Touch): greater than the mid-sized devices (264, iPad) and greater than the larger devices (220, MacBook Pro).

                                                                                                                                              --------Wikipedia