WHAT REALLY IS RETINA DISPLAY IN iPHONE 5?

Late Steve Jobs mentioned an all-new "Retina Display" for the iPhone 4. But what exactly is a "Retina Display?" Apart from a marketing buzzword, it's essentially a high-end screen with four times as many pixels in the same screen real estate.
 

This is what Apple say in one of it advertorials about the Retina Display capability in the newly released iphone 5: “This isn’t just a larger display. It’s a larger Retina display. At 326 pixels per inch, it has a pixel density so high your eye can’t distinguish individual pixels. And as stunning as the Retina display is on the iPhone 4S, this one gives you 18 percent more pixels for an impressive 1136-by-640 resolution. Colors get a boost, too, with color saturation that’s 44 percent greater than before. So with iPhone 5, the games you play, the words you read, the images you see, and the apps you love look and feel incredibly vivid and lifelike. For big-time entertainment, iPhone 5 lets you watch widescreen HD video in all its glory — without letterboxing.”

...It's a screen...

A fancy screen, with a 326dpi resolution. Jobs said "there's a magic number around 300dpi, if you hold something about 10-12 inches away from your eye, it's the limit of the human retina to distinguish pixels".

All the breakthrough technology in iPhone 4 was situated between two glossy panels of alumino-silicate glass - the same type of glass used in the windshields of helicopters and high-speed trains. Chemically strengthened to be 20 times stiffer and 30 times harder than plastic, the glass is ultra durable and more scratch resistant than ever.

.. DPI, Retina and Resolution..

Digital content creators are abuzz about “high DPI” and “Retina” displays in terms of how they are changing the nature of the web and complicating the design process. Before telling you how to develop images for these new devices, it makes sense to understand the basics of the technology: what is high DPI, and why is it significant?

Very simply, “high DPI” (dots per inch) and “Retina” mean the same thing, that is: a device with a high pixel density. “Pixel density” is the number of pixels a display can fit into a fixed distance. This is different from “resolution”, which is a simple count of the number of pixels across the entire width and height of a device.

For example, the resolution of an iPhone 4 is 640 x 960 pixels. All of these pixels – a pixel being the smallest dot of color that is possible to show on a screen – are crammed into a display that is two inches across. 

If we compare that to an old VGA desktop monitor, the monitor will have a similar resolution, but a much larger physical size, and thus a significantly lower pixel density. 

If we divide the physical width of the display by the number of pixels displayed horizontally, the result is the number of pixels per inch (ppi, also commonly referred to as dpi). 

Most current desktop monitors display around 96 to 110 DPI, with laptops coming in slightly higher. “High DPI” is generally acknowledged to be any device with a display density of 200 pixels per inch or greater. 

...and it's close to the iPad's display quality...

The iphone 5 got 78 per cent of the pixels of the iPad with about 326 pixels per inch, but in a far smaller screen-size. That should mean colors will pop, and video, browsing, app-playing—whatever else you do with your iPhone—will be clearer than ever. This will be especially beneficial with fonts, whether they be in emails or when reading in the browser.

...but it's better than your iPhone 3GS' screen...

An 800:1 contrast ratio doesn't say much—you know how misleading contrast ratios can be—but it sounds fantastic for a phone. It boils down to just how they measure it, but we do know that it's 4x better than the 3GS' contrast ratio.

In reality, “Retina” displays are not as far as the technology can be taken: to paraphrase physicist Richard Feynman, there’s still room at the top for further improvement, although it is likely that subsequent advances will be more incremental in nature. It should also be noted that there is a similarly revolutionary leap in color range that is waiting in the wings. 

Once you understand pixel density, the issue is how to optimize your images for the new displays. For bitmap images, the most important point in this new web development process is one that I’ve continually emphasized here – always retain the very highest resolution version of your images.