[Nexus 5 General] danarama:
I can see you're still a bit confused about your argument. PPI is not the unit of measure you should be using. It does nothing to tell us resolution OR screensize so as a number on it's own is entirely useless.

PPI is just a result of resolution and screen size

PPI = width squared + length squared, divided by the square root of screen size.

Optimal viewing distance is about resolution and screen size. The bigger a screen, the further away it needs to be for the resolution to be correct. You can't say "330ppi is optimal" because that's not how it works. You can have 2 completely different resolutions with the same PPI.

For example, on a 5" screen, 720x1500 would be 330 PPI.

Similarly, on a 10" screen, 1500x2950 would be 330 PPI.

The optimal viewing distance of both these example screens is completely different so to say that 330PPI is optimal for a distance is wholly wrong. You cannot use PPI as a valid unit of measure unless you first specify a screen size. There is no such thing as optimal viewing distance for a PPI.

Optimal viewing distance is about resolution and screen size. If you stick to 1 resolution (lets use 1080p as the obvious example), the optimal viewing distance of a 5" screen is less than the optimal viewing distance of a 10" screen. This is because the pixels are bigger on the 10" screen. if you compare an image on a 10" screen to an image of a 5" screen at the same resolution AND distance, and at the opwholeytimal viewing distance of the 5" screen, the 4" image will be crisp and share where as the 10" image will be blocky. You have to increase the viewing distance of the 10" screen to get the same crisp image.

wholey

You can practice this yourself. Put a 1080p movie on your Phone and TV. Watch the movie on your phone at 5" from your face. Watch teh movie on the TV at the same distance. Forget the fact you cannot see the entire screen. Focus on the centre of the screen. You will be able to pick out individual pixels and the section of the screen you are looking at will not appear as a single image. Just a series of blocks. The bigger your TV, the more noticeable this will be.

To get a 330 PPI value on a 4.95" screen like the nexus 5, and maintain 16:9 aspect ratio, you'd be looking at 1450x816 resolution. There are ways to work out optimal viewing distances, but it's quite complex. If you're interested, you can find that information here http://en.wikipedia.org/wiki/Optimum_HDTV_viewing_distance

VD: Viewing distance

DS: Display's diagonal size

NHR: Display's native horizontal resolution (in pixels)

NVR: Display's native vertical resolution (in pixels)

CVR: Vertical resolution of the video being displayed (in pixels)

The optimal viewing distance for a screen the size of a nexus 5 is 7.2 inches (0.6 foot) - which is about what we hold it at.*

*based on:

http://www.calculatorpro.com/calculator/tv-screen-size-calculator/

http://myhometheater.homestead.com/viewingdistancecalculator.html

Now I suppose when we get up to 2k and 4k, there will be a problem. At that distance, the human eye may fail to recognise a difference between 1080p and higher resolutions. To get benefit, the screen would need to be bigger and further away...although some say for those holding the screen right up close to their face, characters will look better

All that said, I can recognise a difference between 720p and 1080p at that distance. So yeah, 1080p is about for a screen our size. Or also for a tablet, which typically would use ant a slightly further way distance than a phone.

So really, it all depends on your eyesight or how short your arms are. If you hold the phone too close to your face, 1080p is not optimal on a 5" screen. You would probably want a smaller screen or a higher resolution, otherwise it may appear blocky. At the appropriate distance for a mobile phone, anything over 1080p may be pointless but bringing it closer may prove beneficial, but that could cause eye strain as you're focusing too close.

There is a really useful chart here

In summary:

If you hold your phone further than the optimal distance, you may get away with losing some resolution as the further away it is, the less you'll notice. OR you need a bigger screen to make it the optimal viewing distance for that resolution

If you hold your phone closer than the optimal distance, you could need to get a higher resolution OR a smaller screen

Like TV's, you should buy the size or resolution based on your fixed viewing distance. For a TV, its the resolution standard of 1080p (because most of our content is that resolution) then you look at the distance your sofa is from the TV stand and buy the correct size. Assuming you need the same resolution for content on your phone, you look at the distance and buy the appropriate screen size. People who hold their phones closer will want a smaller screen for 1080. People who hold them further away will want a bigger screen at that resolution. This trend will continue as content resolution increases, but this cannot go on forever.