Say that again, please

Before we move on, I want to come back to some numbers I threw at you in my last post -- numbers that may warrant a closer look.  I told you that each human retina has about 125,000,000 "sensors" -- either rods or cones.  Did the magnitude of that really sink in?  

Just how big is a human retina?  A typical diameter for an adult human eyeball is about 25mm.  Of course it is not a perfect sphere but let's assume it is.  From various anatomical drawings, the portion of the eyeball lined with the retina starts at the back, of course, and seems to stop at about 70% of the back-to-front distance (the diameter, excluding the bulge of the cornea).  So if I did my arithmetic correctly, an adult human retina is almost 480 square millimeters.  (I ignored the optic disc area that is slightly less than 3 square millimeters.) 

What do these numbers mean in terms of common things?  The eyeball is about the diameter of a U.S. Quarter Dollar.  The area of one retina is about the same as the area of a regular U.S. postage stamp. 

OK, so we believe that there are 125,000,000 sensors within the area of a postage stamp?  That sounds very impressive but how does that compare to similar everyday items?  Let's compare the human retina to the "retinas" of two high-end digital cameras:  Canon's EOS 1Ds Mk3 ($7000) and Nikon's D3X ($8,000).   Both cameras have about the same number of sensors (between 22 and 25 million).  Because both cameras comply with the photographic standard for "Full Frame” images, the "retinas" for both have exactly the same surface area, about 860 square millimeters.  

Result:  Nikon- 28,000 sensors per square millimeter; Canon- 25,000 sensors per square millimeter; either of your eyes- 262,000 sensors per square millimeter.

And you received your two retinas for free.  Take good care of them!

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