Ch. 18  The Eye

 

Main Optics

 

In principle, there is little difference between the basic optics of a camera and the human eye. Both contain a method of focusing light, a way to “stop down” the incoming light to adjust the illuminance, and so on. However, the way that the light is detected is very different.

 

In the case of the eye, most of the focusing is actually done by the cornea, while the lens is for “fine adjustment”. The muscle system surrounding the lens, the ciliary muscle, allows the thickness of the lens to vary. In a relaxed state, the lens is thinner, and objects in the distance are focused on the retina where the rods & cones reside. To accommodate closer objects, the ciliary muscle contracts and thickens the lens.

 

 

In many people, however the lens does not focus the image at precisely the right distance from the lens. In individuals who are “nearsighted” the image focuses in front of the retina. To push the focus back to the retina requires the object distance to become smaller. For farsighted people, the situation is reversed.

 

The vision of farsighted (above left) and nearsighted (below left) eyes can be improved using corrective lenses.

 

In each case, corrective lenses, such as eyeglasses or contact lenses, can compensate for this problem. Laser surgery can also be used to adjust the shape of the cornea.

 

In most cases, the elasticity of the lens decreases with age, so that even with corrective lenses, distance accommodation decreases. This requires the use of corrective lenses with varying focal lengths. Usually this is achieved by changing the curvature at different locations on the lens. The curvature throughout most of the lens is set for distance viewing, that at the bottom for reading (“bifocal”) and nowadays the region just below center for computer terminals (“trifocals”) and similar medium-distance viewing. With today’s lens-making technology, these are “lineless” lenses with smooth transitions among the regions. (There are even bifocal contact lenses with different center and edge focal lengths).

 

Given the very short focal length of the human eye, it is interesting that it suffers from less spherical aberration than a normal lens. The cornea itself has a shape that minimizes spherical aberration by thinning out near the edges.

 

 

The Retina

 

The light-sensing retina has a multilayered structure. In addition to the rod and cones that actually detect the light, there is a network of “hardware” designed to get that information to the brain for processing. This “hardware” is a set of neurons that sits in front of the rods and cones (i.e. in the path of the light!) in much the same way as the electronic circuitry of a front-illuminated CCD. [Personally, if I had designed the eye, I would have put it in the back!]  Then, to get the neural network connected to the brain, they come together in the optic nerve, which must pass through the retina to the back of the eye. Where the optic nerve comes into the eye, there are no rod or cones, but instead there is a blind spot. Because the blind spot is to one side of “center”, each eye “covers” for the other one somewhat. Find it by closing one eye, put up your index finger at arm’s length and move it to the outside about 7 inches.

 

Left: the structure of the rods and cones. Right: photograph of rods seen from the side.

 

The greatest concentration of light receptors occurs in the fovea, a tiny indentation in the retina along the optic axis. This is where most of our high-acuity vision and color perception occurs. It is almost entirely cones here. Within the fovea is the macula, where cones are packed very tightly.

 

 

 

 

 

As one goes away from the fovea, the relative density of cones versus rods changes appreciably. The cones are highly concentrated near the fovea. The greatest density of rods, however, occurs between 10° and 15° “off axis”, which leads to an interesting phenomenon. If you want to see something better at night, it is often possible to do so by looking slightly away from the object! Astronomers often use this “averted vision” to their advantage, either when looking through the eyepiece of a telescope, or simply gazing up at the night sky.

 

 

Inside the rods and cones are layers of membranes containing the actual light-sensing pigments. In the rods, the molecule is a retinine molecule called neoretinene-b, joined to a protein called an opsin to form rhodopsin. In the cones a set of 3 different retinine-opsin combination produces a slightly different molecule, iodopsin. Each combination produces a different pigment that acts as a colored filter. Depending on which one a cone contains, it will be sensitive to red, green, or blue light.

 

The wavelength sensitivity of the three different types of cones. Note the proximity of the peaks of the red and green cones compared to the green and blue. Look back on the C.I.E chromaticity diagram. This demonstrates why saturated green to blue-green is so difficult to achieve for human vision. (Again, I would have done it differently…….)

 

The absorption of a photon by the rhodopsin (rod) or the iodopsin (cone) causes the effective rotation of one of the chemical bonds, breaking it loose from the cell membrane and initiating a nerve impulse. The rods are, overall, about 5 times more sensitive to light than the cones, so that they tend to be used for night vision (with no color discrimination), while the coned dominate at higher light levels.

 

Finally, the nerve impulse is transferred to the brain by way of a neural network.

 

Notice that there are many interconnections here. The signals are transferred outward by bipolar cells (B) to the ganglia (G) that form the base of the optic nerve. Horizontal communication also occurs via the Horizontal (H) and amacrin (A) cells, which are shaded in this figure for clarity.