Common Vision Defects

Effect on Accommodation

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 nearsightedness and farsightedness 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.

What is 20/20 Vision?

My understanding is that the old 20/20 standard was based on being able to read 20 mm high letters at 20 meters distance. The angular separation of two points of light 20 mm apart at 20 meters is

The Rayleigh criterion for diffraction-limited vision for an iris diameter of 5 mm and a wavelength of 500 nm is:

So 20/20 is about 8x the Rayleigh criterion. Ackerman reports that the data show: