How does the formula for retina work?

[Starfyre]

What are the actual mathematics behind the "retina" display to make the formula what it is and how does it works from the keynote?

 

[thetorque]

When you look at a point light source through a hole (you eye is a hole), the image on your retina is not really a dot but a bit spreader out due to diffraction of light. This will make you see a bit larger than the "ideal" dot. When the pixels on the screen are close enough, then the images from each pixel on your retina will overlap and make you not able to resolve those pixels.

Take a look at http://en.wikipedia.org/wiki/Fraunhofer_diffraction