The Human Eye and Lens Systems

The human eye, like many things in nature, is quite a remarkable thing. For many people, our unaided eye can see objects that are both near and far is fairly good detail, and can focus to provide us sharp images in times less than a second. The major part of human vision is reliant on the lens in the eye. The lens is the ellipsoidal disk that lies just behind the cornea. Even though most of "light bending" is done by the air/cornea interface, it is the lens that allows us to see the majority of the detail we see.

A "Perfect" Eye

To begin our discussion on human vision, we will start with a description of what the "perfect" human eye should be. At "rest", a human eye should be able to focus at an image that is coming in from "infinity". While this may sound strange, all this means is that the light rays bouncing off an object are moving in a direction that is almost parallel to one another. In reality, this is actually true for anything that is more than five meters away from our eyes. These parallel rays from an object should be able to focus at the back of our eyes with a minimum of trouble.

In the above picture, there is a scale "reproduction" of the eye. It turns out that the while each of the many components of the eye contribute something, there are two major portions. The first is actual air/cornea interface at the front of the eye, and the second is the lens. Most of the liquid of the eye is at a constant index of refraction, so that has been held constant for the picture, and is represented by the blue. The floating grey disk is the lens, and is arguably the most important part of human vision. The lens can fine-tune our vision, so that we can see objects that are closer than five meters away. The most noticeable thing that occurs for objects in close is that the angles of the incoming ray change. If we did nothing to our eyes, the rays would not focus, and we wouldn't be able to see.

In order for us to see objects are close distance, there are tiny muscles attached to the lens that stretch it out and thin out the lens to reduce the focal length. In the first picture, you can see that the rays do not focus to a complete point at the back of the eye; it seems that the rays still have a little way to go before they focus together. After the lens has been stretched out, then rays will focus at a much shorter distance, and at some point should focus directly on the lens.

In this case, the lens is of the appropriate thickness such that the rays coming in from the point focus altogether. Sadly, this is one of the flaws of using "real scale" modelling. In real life, the change in our lens is incredibly small, and in this case it in no different. If you go to the Focal Lengths and Distances page, you will be more able to clearly see this effect.

Colour Vision
Colour Math
Focal Lengths and Distances
GRIN Systems
Human Vision
Vision Problems