The Retina:

  The retina is made up of two kinds of photoreceptors, rods and cones:

CONES:  The cones are mostly located in the fovea, which is the center of our field of view.  The image of whatever we look directly at falls on the fovea and is detected by the cones.  This is the region of highest acuity, where we see most detail in bright light.  Cones also allow us to see color.  The fovea is about 0.5mm in diameter, which translates to an angular field of view of about 2 degrees.  There are about 5 million cones in the eye, 10,000 of which are located in the fovea.  They are about 1-4 microns in diameter.

RODS:  The rods are all located outside of the fovea, and account for low resolution vision at night.  It is interesting to note that the rods do not see color--in very low light, we do not see color!   There are about 100 million rods in the eye.

  Signals from the rods and cones leave the eye through the optic nerve.  There are no rods or cones in this region, hence no vision.  This is accounts for our blind spot in each eye.  The optic nerve bundle has about 1 million "wires" to accommodate more than 100 million sensors.  Rods are "multiplexed," cones are not.  The optic nerve carries a data rate of about 10 million bits/second.

Electron micrograph of the back of a retina.  Light is incident from behind this picture.

Electron micrograph of rods.