Optics

                  Optics

Myopia and Hyperopia

A myopic eye just means a “nearsighted” eye. If we draw a picture of this eye, we see that it looks big (and long) and that light focuses not onto the retina, but in front of the retina within the vitreous jelly! To correct this refractive error we use a minus (concave) lens to diverge the incoming rays of light. This effectively weakens the overall refractive power of the eye and pushes the image back onto the retina where it belongs.

Hyperopic eyes are small, short eyes. The axial length of these eyes is so short that light focuses behind the eye. To get that image onto the retina we have to add power to the overall refractive power of the eye by using a plus (convex) lens. These convex lenses are basically your traditional magnifying glass and can make your patient’s eyes look enormous at high power.

Near-reading and Presbyopia
Once we get a patient corrected for distance vision we need to take care of close-up vision. With distance vision, the incoming light rays are coming in parallel before entering the eye. A near object, however, produces expanding divergent rays of light. When these rays hit the eye they end up focusing behind the eye.

To get this near object in focus the eye needs some more refractive power. Fortunately, we are born with the natural ability to increase the strength of the lens by making it rounder. This morphing process is called “accommodation.”

The lens works because it is suspended like a trampoline by surrounding zonular fibers. These fibers attach 360 degrees around the lens and tether the lens to the surrounding ciliary muscle. When the ciliary sphincter contracts the zonules relax and the lens becomes rounder. This rounding of the lens increases its magnification/refractive power and allows us to see near objects. With age, the lens becomes dense and does not easily round out. This presbyopia presents after age 40 and progresses with age, explaining the need for near-reading glasses in this age-group. 

Astigmatism
The cornea surface provides the majority of the refractive power of the eye. In the examples above we assumed that the cornea surface was perfectly spherical like a basketball. However, many patients have some degree of astigmatism, where the corneal surface is shaped more like a football. Thus, one axis of the cornea is steeper than the other.