Common Vision Defects

Effect on Accommodation

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.

Numerical data showing the effect on accommodation of 1 m^-1 deviation from normal lens strength.

Simplified Accommodation Model

Though the real eye is a two-lens system operating into a water-like medium (see scale model eye), it is instructive to model accommodation with a single 50 diopter lens treated as if it were operating in air. If this simplified eye is forced to accomodate from infinity to 25 cm it can give some insight into the nature of nearsigntedness and farsigntedness and their corrections.

Focusing at infinity with a 50 diopter lens gives 2 cm as the lens-to-retina distance. Using the thin lens equation and an object distance of 25 cm yields a required accommodated lens strength of about 54 diopters, an 8% accommodation range.