Developing a sound understanding of optics is not only necessary prep for your OKAPs, it’s a fundamental element of your profession. Here’s a quick primer that will go a long way in helping you to understand the optics involved in vision correction and determining a glasses prescription.
A diopter (D) is the unit of lens power equal to 100/focal length of the lens in centimeters or 40/focal length of the lens in inches.
We work with diopters in quarter units, i.e., 0.25, 0.50, 0.75, 1.00, etc.
Corrective lenses are positive or negative:
- Negative (minus) lenses cause light to diverge.
- Positive (plus) lenses cause light to converge.
This term indicates the patient has no refractive error. In general, an emmetropic eye is estimated to be 22.6 mm in length and have 60 D of refractive power.
This means the patient has a refractive error, including myopia, hyperopia and astigmatism.
Myopia — In myopia, the image focuses before it gets to the retina. Negative (minus) lenses are used in the correction of myopia (a.k.a. “nearsightedness”). Negative (concave) lenses are used to diverge the light in order to move the focal point back to the retina.
- Refractive myopia occurs when the power of the eye exceeds 60 D and the length of the eye is 22.6 mm. This is due to steeper corneal curvatures or higher lenticular powers.
- Axial myopia occurs when the power of the eye is 60 D but the eye is longer than 22.6 mm.
Every millimeter of axial elongation causes approximately 3 D of myopia.
Hyperopia — In hyperopia, the image focuses behind the retina. Positive (plus) lenses are used in the correction of hyperopia (a.k.a. “farsightedness”). Positive (convex) lenses are used to converge the light in order to move the focal point back to the retina.
- Refractive hyperopia occurs when the power of the eye is less than 60 D and the length of the eye is 22.6 mm.
- Axial hyperopia occurs when the power of the eye is 60 D but the eye is shorter than 22.6 mm.
Astigmatism — This is a condition in which the eye is shaped more like a football than a baseball. Astigmatism results in two focal points.
This occurs when the natural focusing ability of the intraocular lens is reduced to the point where additional plus power is needed for near tasks. For patients with near-vision problems, especially those 44 years and older, you should expect to prescribe a reading correction.
The estimated reading power at age 44 is about +1.25 D. Add +0.25 D for every two years of age. Therefore, a 50-year-old, for example, would require +2.00 D.
Lens shape varies based on the condition. They may be:
- Spherical like a baseball to correct for myopia or hyperopia;
- Cylindrical like a can to correct for astigmatism or
- Sphero-cylindrical like a football to correct for myopia or hyperopia with astigmatism.
This objective testing technique allows the clinician to determine the patient’s spherical and cylindrical refractive error as well as the axis of his or her astigmatism. It also allows the clinician to determine the refractive state of preverbal and nonverbal patients of all ages and allows visualization of anterior segment opacities or optical irregularities.
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About the author: Ed Hu, MD, PhD, is a refractive cataract surgeon at Illinois Eye Center in Peoria, Ill. After graduating from the Massachusetts Institute of Technology, he received both his MD and a PhD in retinal electrophysiology from the New York University School of Medicine. He completed his residency at the University of Iowa Hospitals and Clinics.