In our discussion of camera resolution we never asked what the resolution of the human eye is?  This is an important question, since ultimately, when we look at a photograph, regardless of how it is presented to us, we are looking at it with the human eye.

Doing the same kind of analysis that we have done to determine the diffraction limit of a camera lens, it can be shown. The human eye has an angular resolution for green visible light of about 1.2 arc minutes.  An arc minute is 1/60th of a degree.  Those pesky Babylonians, with their base 60. are at it again! Physicists prefer a different unit of measure, called the radian.  There are 2 π radians in 360 degrees; so 1.2 arc minutes is about 2.1 milliradians.  The value of using radians is that the spatial resolution is just the distance away from what you are looking at times the angular resolution in radians.

So say you are reading a book or looking at a photograph 12 inches in front of your face, then you resolution is going to be 12 inches X 2.1/1000 = 0.0252 inches.  Remember that this is in line pairs.  There are about 40 line pairs per inch – or 80 lines (or dots) per inch.  This is kissing close to the 72 dots per inch standard that Adobe Photoshop and historically topography use.

Similarly, as I write this, I’m looking from about 18 inches at a 15 inch laptop screen.  So in that case my eye’s resolution is going to be 18 inches X 2.1/1000 = 0.0378 inches or 26.5 line pairs per inch which is 53 dots per inch.  Across my 15 inch screen that’s 794 dots.  If I decide that I’m going to peer in, putting my nose to the screen at about nine inches, I’m going to need twice as many dots per inch or 1,588.  That’s pretty close to the 1366 that my screen is set at.

We’ve seen these numbers before.  But now we realize that the requirements in dots per inch for computer displays and digital prints of various sizes ultimately are defined by the resolution of the human eye.  And hidden in all of this is another important point that the print or display resolution required is defined by how far away you are viewing it.