That maybe true for some people but that's a pretty bold claim for everyone based on your vision.
20/20 is considered "normal". It's also the BARE MINIMUM for fighter pilots. I have 20/15 (well I did 10 years ago probably worse now). So unless you have the best vision of anyone ever then it's a moot point having a statistic of 1.
HTC one like I mentioned earlier looks sharper to me. And maybe it is a placebo to some extent maybe not. Regardless it looks better due to either pixel density, colors, and/or size.
The current ppi on a iPhone is more then adequate for me, but that doesn't mean it is for everyone or it couldn't be better.
News flash: if you're nearsighted things are actually REALLY CLEAR up close. Fighter pilots need 20/20 to have good vision at a DISTANCE. So unless you're holding your phone 20 feet away it doesn't really matter what your vision charts at.
Holding the two phones at a normal distance you absolutely cannot say one has greater pixel density than the other. It only comes into play when you're practically touching your nose to the damn thing and if you're stressing about individual pixels at a proximity you'll never actually be OPERATING it at then you're falling victim to a "spec battle" rather than actual usage.
If you make the PPI higher, all it's going to do is make things look a tiny bit clearer when you press your face against it. It is absolutely NOT going to help with clarity AT ALL when it comes to holding it at a normal distance. Remember the PPI is actually higher on both devices than the MBPr (the 15 has a PPI of about 220, which is around HALF of the S4). Are you going to tell me the MBPr should DOUBLE its density?
It's placebo effect. Put the two phones on a table or have someone hold them at a normal distance and there is no way to differentiate it. All that is different is the color quality (the S4 has more "pop" IMO, which makes it look prettier).
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Btw there is a problem with resolution.
iOS is not Android, they have defined resolutions available , like 640x960 (iPhone 4/4s) , 640x1136 (iPhone 5/5c/5s), 1024x768 (iPad 2/Mini) and the latest 2048x1536 (retina iPads).
What resolution are they going to use in a 4.7" iPhone 6 (for example) ?
"Retina" refers to a given pixel density at a specific usage distance. On phones it's about 320, on laptops around 220. So a 4.7" phone would have whatever resolution would give it 320ppi. Simple as that. As Steve Jobs once pointed out, when you have a device 10-12" from your face, you can't see pixel density any higher than about 300.
A 4.7" diagonal would be 321ppi if they used 800x1280 (standard laptop resolution turned 90 degrees), which is Retina level.