title: What is the formula to determine if a screen is retina resolution?
body: <p>If the iPhone and iPad are both retina display, where retina display means resolution sharp enough to be indistinguishable to the human eye, is there a way to tell if other tablets are retina display quality? Is there a formula?</p>
  
  <p>For example, to be retina quality at 3.5" you need to have 326 PPI correct? Given the width and height and / or screen size in inches and PPI how can you determine it? </p>
  
  <p>For example is Nexus 7 retina quality, is the Samsung Galaxy SIII retina quality, is the Playbook retina quality etc. </p>
  
  <p>More context:
  I'm using a iPad 3 for work and it's resolution is compelling. I like Android as well and want to know how to figure out if other tablets (at whatever size they are) can match the quality. I want to know the minimum DPI for any given size for personal use and development purposes.  </p>
  
  <p>~~ <strong>UPDATE</strong> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<br>
  After reading the post given in the comments I realized the PPI <em>is</em> itself a constant value useful by it's self. Distance does come into the equation but I'm not sure how yet.</p>
  
  <p>Quoting a commenter at that <a href="http://www.tuaw.com/2012/03/01/retina-display-macs-ipads-and-hidpi-doing-the-math/#aolc=BHm3SQ" rel="nofollow">URL</a>,</p>
  
  <blockquote>
    <p>...I also use a
    formula to calculate the distance at which the eye can no longer
    distinguish pixels. Perhaps I'm biased, but I think this formula is
    more practical. With the formula I use, pixel density (ppi) is the
    only information that is necessary. </p>
  </blockquote>
  
  <p>I'm not great at math but I think the forumula would be something like:</p>
  
  <p>distance in inches*80=minimum dpi quality</p>
  
  <pre><code>4*80=324dpi
  </code></pre>
  
  <p>So at 10" away the dpi would have to be: </p>
  
  <pre><code>10*80=800dpi 
  </code></pre>