Eye exercise - Pinhole Glasses

How pinholes function. Pinhole glasses (also known as stenopeic glasses from the Greek words for "little opening") are not made of glass at all but of an opaque substance such as metal or plastic. The user looks through any of the many small holes in the material. These holes have the effect of reducing the width of the bundle of diverging rays (called a "pencil of light") coming from each point on the viewed object. Normally, the full opening of the pupil admits light. It is the improper bending of the outermost rays in that pencil of light which causes refractive errors such as myopia, hyperopia (farsightedness), presbyopia (diminished focusing range with age) and astigmatism to be noticeable. Pinholes can bring about clearer vision in all these conditions. By blocking these peripheral rays, and only letting into the eye those rays which pass through the central portion of the pupil, any refractive error in the lens or cornea is not noticed as much. The pupil may be wide open, but only the central portion is receiving light. The improvement in visual acuity can be striking.

Look at this drawing of a myopic eye with a pinhole lens in front of it. You will see that the eye does not need to deal with the rays that would need the most bending since they are blocked by the lens. It is said that Scheiner first described this effect in 1573. Those who are familiar with cameras will recognize that this is the same principle used to increase the depth of focus by decreasing the aperture. Pinhole cameras also operate by this principle.



An easy way to demonstrate this is to make a fist and put it up to one eye while closing the other eye. Open the fist just enough to create a small hole to look through. If you have a refractive error, you should see more clearly this way. This same improvement in the vision takes place when someone squints to see more clearly. The upper and lower eyelids cut off the rays that would normally enter the top and bottom of the pupil and the vision improves somewhat. Since there is no similar way to cut off rays entering the sides of the pupil, these rays still contribute to the blurred vision. Looking through pinhole glasses instead of squinting cuts off the peripheral rays from all sides. Since the glasses are so close to the eye, the material between the holes is greatly out of focus and is not as disturbing as one might think. After a period of getting used to the glasses, the brain tends to ignore the presence of the material.

Also of interest is that the farther away the viewed object is, the less the pinholes are noticed. The honeycomb effect of the holes is more noticeable when viewing a book held close to the eyes, because the eyes are focused just a short distance in front of the glasses. When looking at a distant TV, however, the holes are hardly visible at all since the eyes are focused much farther away. Also, because of the distance, you can view the entire TV screen through one hole, an obvious benefit.

Looking through the teeth of a comb held in front of the eyes is another way to simulate the pinhole effect. The native people of Alaska have long used this principle by wearing glasses with narrow slits to look through, thus blocking out much of the glare from the sunshine reflecting off the snow and ice. Anyone who remains in this environment for long periods during the summer without protection can suffer from snow blindness. This painful condition forces the person to discontinue the use of the eyes until healing can take place. This is nature's way of protecting the eyes from permanent damage.