Anatomy & Refractive States of the Eye
Principal Ocular Structures

But lets get back to the light we’ve been following. After passing through the aqueous humor, the light passes through the pupil and then to the crystalline lens. As you can see, the crystalline lens is located directly behind the iris and contains about 20 diopters of refractive or focusing power. It functions to bring rays of light to focus on the retina. Suspensory ligaments, hold the lens in place and as indicated on the diagram, are attached on the other end to the ciliary body. The crystalline lens is normally elastic and flexible, with the ability to get thicker and thinner. As the lens gets thicker, its refractive power increases. As it gets thinner, the refractive power decreases. When the eye focuses on an object up close, as when reading, the crystalline lens gets thicker since more refractive power is needed for that activity. The thickening of the lens is accomplished by the relaxation of the ciliary muscle to which it is attached. This phenomenon is known as accommodation.

Referring to Table 1 below, accommodation, we can see that when accommodation occurs it is also accompanied by a convergence of the eyes and a constricting of the pupils. Stated another way, when the crystalline lens gets thicker the eyes come closer together and the pupils get smaller.

ACCOMMODATION
1	The ciliary muscle relaxes allowing for a thickening of the crystalline lens
2	The eyes converge
3	The pupils constrict

But how much do the eyes converge? Well that will vary with the individual, and it will also depend on how far apart the eyes are when accommodation is relaxed. The distance one eye is apart from the other, is known as interpupillary distance or PD. Most distance PD’s fall between about 62-66 mm. If this is the case one would generally subtract 3 mm for near vision when the eyes accommodate. If a PD is relatively narrow, say less than 62 mm, only 2 mm is subtracted for near vision while wider PDs will converge about 4 mm. Now these are just rules of thumb and will be accurate most of the time. However, in many cases such as in higher powered prescriptions or progressive lenses, separate measurements for both distance and near PD’s should be taken. This can be accomplished with a PD millimeter ruler or with one of the various pupilometers available.