The ideal refractive state of the eye is emmetropia 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. An eye whose refractive power does not produce this precise focus is ametropic and is described as having a refractive error. 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. Emmetropia: Parallel rays of light from optical infinity focused as a single point on the retina. Six Principles of Refraction
1. Refraction is the art of improving vision without medicine or surgery.
2. Refraction and prescribing glasses are best approached as problem solving.
3. The process is more than measurement, and what we measure is not necessarily what we give.
4. 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.
5. The goal is to give the simplest system that satisfies that individual patient's visual needs.
6. The appropriate prescription is decided upon for and with each patient. Explain and always show the patient, binocularly, what you will be prescribing for them.
7 Subjective Refraction and Prescribing Glasses:
Guide to Practical Techniques and Principles Snellen Visual Acuity What does 20/20 vision mean? Twenty feet is used for measuring visual acuity because that distance approaches and approximates optical infinity. Thus, light rays from twenty feet away are essentially parallel as they come into the eye. The Snellen, or other, visual acuity chart is placed twenty feet from the patient 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 can see. That individual can read letters of a given size at twenty feet. Their visual acuity is 20/20 because, by definition, they can read at twenty feet what the normal individual can read at twenty feet.
What does 20/80 vision indicate? If someone has 20/80 vision, that means they must be twenty feet away to see clearly what the normal person can see clearly from eighty feet away. Can vision be better than 20/20? Yes, some individuals have better than 20/20 visual acuity.
If, for example, someone has 20/15 acuity, they can read from twenty feet away what the normal person would need to be fifteen feet away from to be able to read. Spherical Refractive Errors 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 myopic or nearsighted. This is corrected with a concave lens — a minus (red) lens — which diverges the rays of light so that the focal point moves posteriorly and is focused on the retina.
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 hyperopic or farsighted. This is corrected with a convex lens — a plus (black) lens — which converges the rays of light so that the focal point moves anteriorly and is focused on the retina Myopia and hyperopia are corrected by a "sphere" of a specific power. 9 Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles. 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 lens diverges rays of light. C. A minus (red) lens corrects myopia.
Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles. Hyperopia: A. Parallel rays of light from optical infinity focused as a single point behind the retina. The eye is "too short." B. A biconvex lens converges rays of light.
C. A plus (black) lens corrects hyperopia. Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles 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 they can be called "near-sighted." 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. But 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 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!
