♦ Optically, the purpose of refraction is to place a focal point onto the retina.
♦ Clinically, refraction is how we determine best corrected visual acuity as well as the optimal glasses prescription.
The ideal refractive state of the eye is emmetropia (Figure 1-1). In an emmetropic eye, the refractive powers of the cornea and the crystalline lens combine to precisely focus parallel rays of light from a distant object onto the retina as a single point. The cornea plays the greater role in achieving this.
Optically, the purpose of refraction is to place a focal point onto the retina. Clinically, refraction is how we determine best corrected visual acuity as well as the optimal glasses prescription; the latter is maximally important to the patient.
The goal of clinical refraction is to determine the strength of the corrective lens that will achieve this precise focus when placed in front of the eye. With a well-performed refraction, we are helping our patients see more clearly—without medicine or surgery—and they benefit from a correct glasses prescription all day, every day!
An eye whose refractive power does not produce this precise focus is ametropic, and is described as having a refractive error.
Figure 1-1. Emmetropia. Parallel rays of light from optical infinity focused as a single point on the retina.
- Refraction is the art of improving vision without medicine or surgery.
- Refraction and prescribing glasses are best approached as problem solving.
- The process is more than measurement, and what we measure is not necessarily what we give.
- History, examination, diagnosis, and treatment decisions are necessary. Just as with medical problems, history plays a large role in determining what will best help the patient.
- The goal is to give the simplest system that satisfies that individual patient’s visual needs.
- The appropriate prescription is decided upon with each patient. Explain and always show the patient, binocularly, what you are considering prescribing.
Note
After successful eye surgery, a patient with residual refractive error is able to achieve the full benefit of the surgery only when fitted with the proper glasses.
What Does 20/20 Vision Mean?
The correction of refractive error is the focusing of parallel rays of light onto the retina. Light rays from optical infinity enter the eye in a parallel fashion, and those from twenty feet are essentially parallel. It is for this reason the visual acuity chart is placed 20 feet from the patient.
Identifying letters on the Snellen chart is the classic, and still the most common, way visual acuity is measured. The ETDRS acuity chart has also been in use since the commencement of the Early Treatment Diabetic Retinopathy Study in 1979.
The Snellen (or other) visual acuity chart is placed at 20 feet either literally or by way of mirrors. Alternatively, the size of the letters on the chart can be adjusted to achieve essentially the same effect.
The 20/20 designation is based on what the “normal” individual is able to see. A person with normal vision is able to read letters of a given size at 20 feet. Visual acuity of 20/20 means that the individual can read at 20 feet what the normal person can read at 20 feet.
The metric equivalent of 20/20 in meters is 6/6.
What Does 20/80 Vision Indicate?
If someone has 20/80 vision, that means the individual has to be 20 feet away to see clearly what the normal person can see from a distance of 80 feet.
Can Vision Be Better Than 20/20?
Yes, some individuals have better than 20/20 visual acuity. If, for example, one has 20/15 acuity, that individual can read from 20 feet away what the normal person could only read at 15 feet.
Is Pinhole Acuity the Same as Best Corrected Visual Acuity?
No, it is not. It is not unusual for refraction to yield better acuity than the pinhole.
There are two types of spherical refractive errors: myopia and hyperopia.
In myopia, parallel rays of light from optical infinity, bent by the cornea and the crystalline lens, come to a focal point in front of the retina. The eye is “too long” relative to its inherent plus power. The patient is described as being myopic or nearsighted. This is corrected with a biconcave lens—a minus (red) lens—which diverges the rays of light so that the focal point moves posteriorly and is focused on the retina (Figure 1-2).
In hyperopia, parallel rays of light from optical infinity, bent by the cornea and the crystalline lens, come to a focal point behind the retina. The eye is “too short” relative to its inherent plus power. The patient is described as being hyperopic or farsighted. This is corrected with a biconvex lens—a plus (black) lens—which converges the rays of light so that the focal point moves anteriorly and is focused on the retina (Figure 1-3).
Myopia and hyperopia are corrected by a spherical lens, or sphere, of a specific power.
Figure 1-2. Myopia.
A. Parallel rays of light from optical infinity focused as a single point in front of the retina. The eye is “too long.”
B. A biconcave (minus) lens diverges rays of light.
C. A minus lens corrects myopia.
Figure 1-3. Hyperonia. A. Parallel rays of light from optical infinity focused as a single point behind the retina. The eye is “too short.” B. A biconvex (plus) lens converges rays of light. C. A plus lens corrects hyperonia. Fallacy Alert There is an interesting fallacy in the seemingly straightforward term farsighted. When not wearing corrective glasses, a moderately nearsighted person has blurred vision at distance but can see clearly at near; thus, being called nearsighted. The term nearsighted works: one can see clearly at near, but not at far. One might then logically conclude that, for a farsighted person, the opposite would be the case—the individual would have blurred vision at near but be able to see clearly at distance. However, this is not so. A moderately farsighted person has blurred vision at distance! Thus, the term farsighted is a fallacy because the individual cannot see clearly at far without correction. The reason this seemingly simple terminology breaks down is that the nearsighted/farsighted designations do not properly describe what is happening with these refractive errors. A camera analogy demonstrates what is occurring in myopia and hyperopia. If a camera is in focus at distance, the image becomes equally blurred if the focusing lens is turned to the right or the left; that is, if the image is defocused anteriorly or posteriorly. Similarly for the eye, a distant image will be blurred if the focal point is in front of (myopia) or behind (hyperopia) the retina. So, should the terms nearsighted and farsighted be replaced, respectively, with “like a camera whose image is blurred because it is displaced too far anteriorly” and “like a camera whose image is blurred because it is displaced too far posteriorly”? Probably not! Astigmatism is a Greek term meaning “without a point.” Parallel rays of light passing through an astigmatic surface do not come together as a focal point. An eye’s astigmatism is mostly due to the curvature of the cornea, although the crystalline lens can also contribute. An easy way to understand astigmatism is to consider the difference between a basketball and an American football. We know that they have different shapes, but we usually do not stop to think about exactly how they are different (Figure 1-4). A basketball has one curvature. Wherever you look, the curvature is the same. A single curvature equates to an optical system that is spherical. Figure 1-4. Astigmatism. A. A “basketball” cornea has no astigmatism. The curvature of the solid line and the broken line are the same. B. A “football” cornea has astigmatism. The curvature of the solid line (more gradual) and the broken line (more steep) are not the same. C. The angle of the football determines its axis. An American football looks different. This is because it has two curvatures. If we put the football on a table and hold it so that its tip is pointing upward, we can see that it has a gradual curve from top to bottom and a steeper curve from side to side. Two different curvatures equate to an optical system that is astigmatic. ♦ A “basketball” cornea has no astigmatism. ♦ A “football” cornea has astigmatism. Thus, a basketball-shaped cornea has no astigmatism, in contrast to a football-shaped cornea, which is astigmatic. Astigmatism is corrected by a cylinder of a specific power and orientation. The orientation is called its axis. There are two ways the power and axis of the cylinder can be determined: the plus cylinder method and the minus cylinder method. Just as a given length can be expressed in inches or centimeters, a prescription for glasses can be measured and recorded with plus or minus cylinder power. A plus cylinder prescription can be converted to a minus cylinder prescription, and vice versa (see Plus-Minus Cylinder Conversion on page 24). Either system of notation can be used for an eyeglass prescription. Minus cylinder is used for contact lens prescriptions and for refractive surgery. Plus cylinder is used in the calculations for toric (astigmatism-correcting) intraocular lenses. Where they differ, the plus and minus cylinder methods are discussed separately. The principles of refraction apply equally to both. Note In contact lens and intraocular lens terminology, the term toric is used to describe a lens correcting for astigmatism. Axis is another aspect of astigmatism that must also be taken into account. We have described the two curvatures of the football while it is held with the tip pointing upward, but a football can also be held in a variety of tilted positions to the left or right. It can even lie on a table in a horizontal position. The orientation of the football is its axis, which can range from 0 to 180 degrees from the horizontal. An astigmatic cornea is described in the same manner. An astigmatic cornea that has not been altered by corneal disease will have two curvatures 90 degrees apart; this is termed regular astigmatism. Regular astigmatism can be corrected with a cylinder in the glasses prescription. If the shape of a cornea has been altered by corneal disease, it will have more than two curvatures; this is termed irregular astigmatism. Irregular astigmatism cannot be fully corrected with a cylinder and, therefore, a rigid contact lens is often used to allow the tear film to fill in the irregularities of the surface. ♦ When parallel rays of light from an object pass through a spherical refracting surface, a single point of focus is the result. ♦ However, when parallel light rays pass through a spherocylindrical system, a single point of focus is not achieved. When parallel rays of light from an object pass through a spherical refracting surface, a single point of focus is the result. However, when parallel light rays pass through a spherocylindrical system, a single point of focus is not achieved. The result is a unique appearing shape, which can be visualized as two ice cream cones joined at their tips, with one cone squashed horizontally and the other vertically. This configuration was first described by J.F.C. Sturm in 1838 and is called the conoid of Sturm (Figure 1-5A). If one cuts cross-sections sequentially through the conoid, the result is a series of elliptical images, two focal lines, and a central circle (Figure 1-5B). The most important parts of the conoid of Sturm are the two focal lines and the circle, the latter always located dioptrically halfway between the two lines. A vertical curvature will produce a horizontal focal line, and a horizontal curvature produces a vertical focal line. If, to simplify the conoid of Sturm, one ignores the ellipses and looks simply at the two focal lines with the circle in the center, this is called the interval of Sturm (Figure 1-5C). If, further simplifying the conoid of Sturm, one eliminates the ellipses and the two focal lines, only the circle remains. The circle is called the circle of least confusion (Figure 1-5D). Thus, the circle of least confusion can be used to represent the conoid of Sturm, the result of light rays passing through a spherocylindrical surface. The circle of least confusion is also referred to as the blur circle. The blur circle is produced by an astigmatic refracting system, in contrast to a point of focus produced by a spherical system. It is always, dioptrically, midway between the two focal lines. The closer the focal lines are to each other, the smaller the blur circle. The blur circle is the clearest image when astigmatism is not fully corrected. Figure 1-5. The conoid of Sturm. A. The conoid of Sturm. B. Cross sections through the conoid of Sturm. C. A simplification: the interval of Sturm. It is the distance between the two focal lines. D. Further simplification: the circle of least confusion. It is always dioptrically midway between the two focal lines. It is also referred to as the blur circle. When on the retina, it represents the spherical equivalent.
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