Here’s a slow motion video showing a closeup look at the human eye, our amazing biological lens (and sensor). You might be surprised at how mechanical its movements are and how fluid the iris is. Another crazy fact is that we’re continually relying on “image stabilization” to see things clearly:
The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. [#]
To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about 1 Hz, the fingers will become a blur. Now, keep your hand still, and shake your head. No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand. [#]
Like with cameras, our built-in image stabilization can deal with head shake but not motion blur.