Refraction

As part of a comprehensive vision examination, along with checking the eye for physical disorders such as glaucoma or dry eye syndrome, your eyecare practitioner will perform a refraction, which is the process that finds the right amount of lens prescriptive power that will be put into a pair of eyeglasses (if you need them). This process is called the refraction; it is usually done with the use of an instrument called a phoropter or refractor, which allows you to look through it while the practitioner changes lenses and asks those “which is better, one or two” questions.

Refractive disorders of the eye such as myopia, hyperopia, astigmatism and presbyopia are measured in units called diopters. Diopters represent the amount of focusing correction needed to normalize your vision; the more nearsighted, farsighted or astigmatic you are, the more diopters of lens power are needed to correct it.

Presbyopia is corrected by an additional amount of focusing power for near vision; it can be prescribed by itself in reading glasses, or be combined with lenses for distance vision in the form of a multifocal lens.

(For the math mavens, a diopter is the reciprocal of the focal length of the lens, in meters. Thus, a two-diopter lens has a focal length of one-half meter.)

Negative powered lenses (minus lenses) correct myopia (nearsightedness), and positive (plus) lenses) correct hyperopia (farsightedness). Lenses that are thicker on the edges than they are in the center are minus, while lenses that have centers thicker than the edges are plus lenses. They are called plus or minus because when you look through them, minus lenses make things look smaller and plus lenses magnify objects.

Almost all ophthalmic lenses have some amount of curvature on both the front and back surfaces. Their total power equals the power of the front surface plus the power of the back surface, added algebraically, taking into account the plus and minus signs. For example, a lens with a front surface of +5.00 and a back surface of -4.00 will make the total power of that lens equal to +1.00. Another example is a lens with a +2.00 front surface and a -5.00 on the back surface; this one has a total power of -3.00. This fact is not really important, except when we get to the topic of neutral-powered lenses, below.

Astigmatism is somewhat trickier, because it can be written in either plus or minus form; either form is valid, and one can be converted into the other while still describing the same ophthalmic lens. For some reason, optometric physicians use the minus form in writing prescriptions for astigmatism, and ophthalmologists use plus notation in theirs. The astigmatism correction may also be referred to as the cylinder, shortened to cyl. This lens, described by the prescription +2.00 -150 X 180 in minus-cylinder form, but can also be written as +0.50 +150 X 90 in plus-cylinder. Either of these two forms describe the same lens.

Myopia, hyperopia and/or astigmatism prescriptions are composed of two numbers. The first one is either a plus lens (+), a minus lens (-) or a zero. This first number is the amount of correction in diopters, for myopia or hyperopia. If the prescription does not call for either, the first number will be zero, representing a lens with neutral power; this can also be written as plano or pl. The second number represents the amount of astigmatism, which is followed by another number from one to 180, which is the orientation of the astigmatic correction, as would be found on a protractor.

One lens can only correct myopia OR hyperopia, not both, but may also contain a correction for astigmatism and/or presbyopia.

If there is no significant myopia or hyperopia to be corrected, the first number will be zero, or plano, followed by the amount of the astigmatism correction and its orientation in the eyewear. An example of such a prescription might be: 0.00 -2.00 X 90. This lens has no myopic or hyperopic correction, but the patient does have astigmatism, and the lens correcting that is oriented straight up-and-down. Someone with myopia and astigmatism might have a prescription of -3.50 -1.50 X 180; meaning, this person needs three and one-half diopters of minus power to correct his or her myopia, and one and a half diopters of power oriented from side-to-side to correct his astigmatism.

• The first number represents the amount of myopia or hyperopia, if any.
• If there is no myopia or hyperopia, the first number will be written as “0.00” or “plano,” sometimes shortened to “pl.”
• The second number is the amount of astigmatism, if any, and will be followed by an orientation number to describe how the lens should be oriented in the eyeglass frame.

Presbyopia and Add Power

Presbyopic lenses are written as add power, because it is power added on to the correction for myopia/hyperopia and astigmatism, so that the person using it can see well up close. If the person needing lenses for reading has no myopia or hyperopia, and no astigmatism, the power of the add may be in the form of single vision reading glasses or as a multifocal lens with neutral power in the top part of the lens so as to provide clear vision in the distance, and the add located in the bottom part of the lens so the person can see to read through it. If the person does have a refractive error (myopia, hyperopia or astigmatism) as described above, the presbyopic prescription will be in the area on the form called the “add.” For example, here’s a prescription for someone who does not need a correction to see clearly in the distance but needs help focusing for his computer: plano with +2.00 add. If the person needs prescription lenses to see clearly in the distance, the add power will be added onto that prescription, as in: --5.00 -2.00 X 180 with a +2.00 add. Usually, eyewear prescriptions are written on forms with boxes labeled Right and Left lenses, and additional boxes for the add power. The right lens will be labeled O.D. (ocularis dextra) and the left as O.S. (ocularis sinistra). A prescription calling for the same lens power in both eyes can also be labeled O.U., as in +2.00 O.U. These abbreviations can be shown with or without the periods between them; they still mean the same thing.

Zero-Power or Neutral Power Lenses

Lenses with neutral power are not “window glass,” but have curvature as well; in this case, the front surface and the back surface curvatures cancel each other out; this can be written as “plano.” Such a lens must still be manufactured by the same process used in any other lens, and it must be surfaced, edged and hardened in exactly the same way. No lens that will be put into an eyeglass frame is ever just “window glass;” Because of this, plano lenses cost the same as lenses that correct refractive errors.

Examples

Here are some examples of eyeglass prescriptions:

• John has a prescription like this: OD: +2.50 -1.25 X 121 +2.25 add

OS: +1.75 sphere +2.25 add

This prescription means John has hyperopia in both eyes, astigmatism in the right eye only, and a presbyopia prescription of +2.25 added to both lenses for reading.

• Phoebe’s lens prescription looks like this: OD: -1.25 sphere

OS: -1.50 -0.75 X 90

This means Phoebe is myopic in both eyes, but has mild astigmatism in the left eye only.

She does not need an add to read with, but can wear her glasses for all distances. (However, she may see very well without any glasses at all if she is reading.)

• Timothy has new glasses with this prescription: OD: plano +1.50 add

OS: plano +1.50 add

Timothy has the option of having his glasses made up as a pair of single-vision (no multifocal) reading lenses of +1.50 OU, or as a pair of multifocals with neutral power in the top parts of the lenses and the add in the bottom.

Multifocal Lenses

Multifocal lenses mean just that: they have more than one power incorporated into the same lens. Having multifocal lenses does not necessarily have anything to do with one’s age or how bad their vision is. A prescription for multifocals or bifocals only means that for the convenience of the wearer, there is one prescription in the top part of the lenses, and a different power in the bottom. Progressive lenses do the same thing, but without the abrupt change that a regular, flat-top style bifocal lens provides.

Often, multifocal lenses are prescribed for children as part of a program of preventing headaches or eyestrain; they may also be prescribed for college students who do a lot of studying or must spend hours in front of a computer screen. Either of these may also be prescribed in an effort to stabilize the amount of myopia by reducing nearpoint stress.

When Benjamin Franklin first cut the lenses of his distance glasses and his reading glasses in half and re-mounted them into the same frame with the reading power in the bottom half, he was only doing it for his convenience. By putting both of his prescriptions into the same frame, he avoided having to switch from his distance glasses to his readers each time he wanted to look from far away to close up.

Curiosity Satisfied

No one needs to memorize all this information, as it is only given as information for those who might be interested in knowing more about their vision. (There will be no quiz later.) Sometimes, knowledge itself is its own reward, satisfying curiosity.

If you have questions about your eyeglass prescription or how it might have changed since your last vision examination, feel free to ask your eyecare practitioner or the optician.