Mission Eye Clinic - Understanding Your Eyeglass Prescription: Common Refractive Errors

Understanding Your Eyeglass Prescription: Common Refractive Errors

The most common cause of blurry vision is undoubtedly the need for an up-to-date refraction/spectacles. (There are, however, many other possible causes of blurry vision. Because a “stand-alone” refraction does not meet clinical standards of vision and eye care, a refraction is always done as one of the components of an eye examination.)

Refraction: Measurement of the refractive error of the eye (or focusing error of the eye) which is used in conjunction with other eye examination findings to determine your eyeglass prescription. Sometimes a refraction must be done under cycloplegia (eyedrops to relax your focusing which also dilate your pupils). A refraction is an integral part of an eye examination; it is usually required to aid in the diagnosis of various ocular symptoms, diseases, and conditions.

Dioptres: Unit of measurement of the refractive error of the eye which is used to determine the optical power of eyeglass lenses needed. These are the positive or negative numbers written on your eyeglass prescription which usually increment in 0.25 dioptre steps.

Refractive Errors: There are four main types of refractive errors: Hyperopia, Myopia, Astigmatism, and Presbyopia. Very often a patient will have a combination of these focusing errors.

Accommodation: The usually involuntary adjustment/focusing of the eye for seeing at different distances; achieved by changing the power/shape of the eye's crystalline lens.

Emmetropia: Ideal refractive state of the eye where there is no myopic, hyperopic or astigmatic refractive errors present.

Myopia: Refractive condition of the eye in which distance objects are focused in front of the retina and therefore results in a blurred image; also known as nearsightedness, shortsightedness. For patients with mild myopia, distance objects will be blurry but near objects are usually clear. For patients with moderate to severe myopia, near objects may also appear blurry unless they are held at an unusually close distance (closer distances for higher myopia).

Hyperopia: Refractive condition of the eye in which objects are focused behind the retina when the accommodation is relaxed; also known as farsightedness, longsightedness. Depending on the amount of hyperopia and a patient's age (and associated accommodative ability), they may be able to partially or fully compensate for their hyperopia by accommodating. Hyperopic patients usually have a more difficult time focusing at near which results in near blurry vision. They can, however, also have a difficult time focusing at distance. Although sometimes it can result in similar symptoms, hyperopia is different than presbyopia.

Astigmatism: Refractive condition of the eye in which the image of a point object does not meet in a single focal point which results in blurred vision; associated with differences in power/curvature of the refractive surfaces of the eye in different meridians. In other words, the eye is shaped more like a "football" than a "baseball", and focuses differently in different meridians. Uncorrected astigmatism causes blurry vision at all distances (near and far). The amount of blur experienced depends on the amount of astigmatism present.

Presbyopia: Refractive condition of the eye in which the accommodative abilities of the eye become insufficient to focus clearly on near objects; due to a normal decrease with age in the ability of the crystalline lens to change power/shape (typically occurs in the forties). This usually results in blurry vision at near and difficulties reading, and occurs in addition to any hyperopia, myopia, or astigmatism you may have. Some patients may experience slightly different manifestations of this condition although it eventually occurs for everyone, for example: patients with mild myopia often find that they can still read but that they have to take off their myopic distance glasses. The amount of presbyopic correction needed (which is called the near add) depends on the individual patient's remaining accommodative ability as well as the distance they are conducting their near visual tasks (e.g., distance they usually hold a book, distance to the desktop, or distance to the computer screen). The closer the near visual task, the stronger the near add required. It is for this reason that patients often report that "their arms are getting too short".

Our doctors and expert dispensers will provide you with their skill, knowledge and professional recommendations to best meet your visual and optical needs. The many types of lens and frame options to correct your refractive error will be explained. This includes single vision, bifocal, trifocal, and progressive (no-line bifocal) lenses. Anti-reflection coatings, UV protection, tinting, photochromic, high-index, and aspheric are some of the lens options that are available. We have a large dispensary with a great selection of frames and our own in-clinic edging lab.

Select one of the following topics for more information about eyeglasses from the Mission Eye Clinic:

>Lens Types

>Lens Features

>Frames