Notes by Dr. Optoglass: The Human Eye – Part I

Topics Covered:

  • A Diagram of the Human Eye for Videographers and Photographers
  • The Pupil, Iris and the Lens
  • The Vitreous Body

Every closed eye is not sleeping, and every open eye is not seeing – Bill Cosby

Start by knowing this: The human eye is more complex than any camera, and possibly more complex than every camera put together. It has great strengths and equally great limitations. For what it’s worth, it has helped mankind see the mundane, the absurd, the great, the horrific and the sublime.

Everything about imaging starts from the human eye. This is what it looks like:

For the sake of videographers and photographers, the eye can be re-imagined like this:

Eye Diagram

A quick look at the above diagram will tell you light passes through a lens (not made and tested in a lab), through layers of gel-like material and finally hits a small spot (that has rods and cones chugging away as long as the eye is open). Wow. This is the moment from which you forever stop complaining about camera and lens design.

What the design of the eye tells us is that the path that light takes to the brain is very complex, and is nowhere as simple as how light strikes a sensor or film in a camera. How can the eye possibly see so well with such a convoluted design? I don’t know, but it does. Very well, thank you.

The Pupil, Iris and Lens

Credit: Petr Novák, Wikipedia

The black hole in the middle is the Pupil. Why is it black? Because the eye sucks up all the light so efficiently inside that none reflects out.

The colorful part – the one that comes in many shades – is the Iris. It is what decides how big or small the pupil should be. When strong light enters the pupil, the Iris shuts it down (If you really want to shut out light completely, you have eye lids). When the available light isn’t bright enough, the Iris compensates by widening the pupil.

The minimum pupil size is about 3mm to 5mm, and the maximum is about 9mm. This varies greatly among humans, depending on age and other factors.

The white outer area is the Sclera, the central transparent part (which you can’t see but is there) is the Cornea.

The lens is suspended in space somewhat, and along with the cornea, refracts (which means passes through) light and helps focus it (what else is a lens for?) on the retina. Unlike camera lenses, this one can change shape depending on how far the subject is. This changing of the lens’ shape is called Accommodation. Why does the lens need to accommodate? Because otherwise the entire eye needs to expand and contract (like bellows on a camera) to focus.

The lens is around 10 mm in diameter and is about 4 mm long in adults – but this changes due to accommodation.

The refractive index of the lens varies from approximately 1.406 to 1.386. So is it made of glass? Not really. It’s mostly made of Crystallins or water soluble proteins. No complaints. If you prefer glass I can prescribe you eye glasses.

Once light passes through the lens it has to travel through a gel-like sea called the Vitreous Body or Vitreous Humor. 99% of it is water. The other 1% is the subject of a lifetime of study. Not surprisingly this 1% gives the vitreous body a viscosity two to four times that of pure water. Its refractive index is about 1.336. It keeps the eye’s shape. If we had air instead of gel, the eye could easily collapse like a ping ping ball.

Takeaways:

  • The Eye is complex enough to demand a lifetime of study on its own.
  • The Pupil is a hole through which light passes.
  • The Iris controls the size of the pupil.
  • The lens of the human eye accommodates (which means it changes shape) to focus. It, with the help of the cornea, controls the path of light.
  • The Vitreous body holds the eye together and still lets light pass through.

Links for further study:

Next: The Human Eye Part II
Previous: Spatial Frequency