Speaking of pedantic, what is the difference between "high resolution screens" and "high DPI screens"? Assuming, of course, that screen and display mean the same thing.

High DPI refers to a high pixel density. High resolution refers many pixels.

If I have one million pixels on a 10cm by 10cm screen, and one million pixels on a 100cm by 100cm screen, they both have the same resolution because they both have the same number of pixels.

However, the 10cm screen has a much higher pixel density because the pixels are much smaller and closer together in order to fit on such a small surface.

The effect of a high pixel density is that you cannot tell pixels apart because they are so small and clustered, even up close. This results in a much more fine looking image.

'Retina' is just a marketing term for high DPI displays.

> High resolution refers many pixels.

That's correct, but only within our narrow jargon.

In optics, when I say "resolution", I mean the smallest distance (either angular or linear) beween two points that can be distinguished by the apperatus. E.g. "The CCD resolution is 10 microns per pixel". Not how many distinct points there are ("The CCD has 1024x786 pixels").

Sometime in the computer age, "resolution" became used for the latter meaning and so DPI had to be used where "resolution" was previously meant.

But don't blame people if they still use the original meaning.

2560x1440 is ~109 dpi when the pixels are arranged to fill a 27" monitor. (This is not considered "Retina".)

2560x1440 is ~500 dpi when the pixels are squeezed into a 6" phone display such as as Google Nexus 6/6P.

The "Retina" description refers to the dpi exceeding 300 dpi and not the absolute # of pixels.

Resolution is how many pixels per the entire screen. DPI is how many pixels per a inch.

For instance, a high resolution 4k TV that is 50" might not be considered high DPI compared to a 4k 15" laptop screen.

"High resolution" could mean any display >= 1080p or so, whatever size (6 inches, or 6 feet).

DPI, being "dots per inch" relates to resolution for a given physical size, so a "low resolution" 720p display that was only 1 inch across would have a high DPI (in this case, 720 dots per inch), but a "high resolution" 4k display at 50 feet across would have a low DPI.

Generally speaking, the higher the DPI the harder it is to discern individual pixels, and the better for viewing fine details.

High resolution screens have lots of pixels and high DPI screens have many pixels per inch.

If you look at an Ultra HD (3840x2180) 40" TV close up versus a 5" phone screen (say 1334x750) viewed from the same distance you'll notice the phone screen is sharper despite having fewer pixels.

Right, my mistake.