The iPad has a bigger screen, and so it has a bigger resolution. With the iPhone 4, however, Apple did something different: they increased the resolution without changing the physical size of the screen. This makes for a higher pixel density, pixels are smaller and more tightly packed. In one square inch of the iPhone 4’s screen there are around 106 thousand pixels (at 326 PPI, or Pixels per Inch), while the older models have only about 26 thousand (at 163 PPI) in an inch – 4 times less! This makes graphics on the screen seem like continuous lines, because your eye can’t see the individual pixels. Apple calls this technology the “Retina Display” because they claim the human eye (the retina) can’t physically see the pixels at this resolution.
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