Optics for Optometry

Taking a nominal 50 diopter lens strength for the eye's lens combination, the effect of just a 1 diopter change in the lens strength upon accommodation can be assessed. It is more dramatic than you might expect with a 1 diopter nearsigntedness taking away clear vision beyond 1 meter and only adding 7 cm to the close-focusing capability. Fortunately, this defect can be corrected with just a -1 diopter corrective lens. The person with 1 diopter of far-sightedness can go for many years without knowing it because they can focus clearly to infinity, and with a nominal close-focus distance of 31 cm, young eyes can likely focus more closely with effort. But with the loss of pliability of the internal lens in later years, they may have eye-strain headaches and reach the point where they cannot read at normal distances.

This depicts the effects of rather extreme cases of nearsightedness, farsightedness and astigmatism on the paths of parallel input rays, which would be characteristic of objects at effectively infinite distance, like looking at the moon. In these extreme cases, neither the farsighted eye nor the nearsighted eye would see the moon clearly. The nearsighted eye refracts the light too much, focusing at a position before it reaches the image-detecting retina. The farsighted eye does not refract the light enough, and cannot bring the rays to a focus by the time they reach the retina. The diagrams also assume a relaxed eye, in which case the focal length of the eye's interior lens is at a maximum. The eye's effort in accommodating to view closer objects involves muscle tension that results in the interior lens being more rounded and therefore of shorter focal length (greater refracting power). In the cases depicted here, accommodation might be sufficient to bring the moon into ...