The miracle of sight is one we hold dear. Maintaining healthy eyes is in everyone’s best interest. If our vision begins to fail, science and medicine play a large role in recreating vision through various optical technologies.
The following guide of Vision Conditions is to help you gain a better understanding of your own visual condition, so that you can better prepare to find the best optical solution to fit your needs. Let’s start with an overview of three main categories, Refractive Errors, Conditions of the Eye, and Diseases of the Eye.
If you have 20/20 vision at both distance and near, you have Emmetropia. An Emmetropic Eye has the ability to focus distant light rays to a point focus on the retina. In addition to this, the Emmetropic eye can accommodate and focus near point objects for reading without the aid of ophthalmic lenses.
Refractive errors, simply put, are errors in the refraction of light through the eye. That is, the eye’s inability to properly transmit and refract light to focus on the retina, in the back of the eye. There are three types of Refractive Errors: Myopia, Hyperopia and Astigmia.
Myopia, sometimes referred to as “nearsightedness”, is a refractive error where light does not reach the retina. Light comes to a focus before the retina, in the vitreous humor, the gel-like substance inside the globe of the eye. Because the light rays do not focus on the retina, the image formed in the brain is blurred. There are three types of Myopia which include Corneal Myopia (cornea is too steep), Axial Myopia (length of the eye is too long), or Lenticular Myopia (the lens system inside the eye is too strong). The myopic eye refracts light too much…it needs minus (concave) lenses for correction.
The far point of the myopic eye, its own unique focal length, is inside optical infinity (inside twenty feet). If an eye has a low amount of myopia, its far point could be out to 10, even 15 feet. If an eye has a large amount of myopia, its far point could be as little as 10 centimeters. In either case, ophthalmic lenses are used for its correction. Myopic eyes have a tendency to increase slightly over the years, and rarely remain stable over one’s lifetime. It is important to visit your eye doctor on a regular basis to be sure that you manage your myopia and eliminate eyestrain with current lenses.
Hyperopia, sometimes referred to as “farsightedness”, is also a refractive error of the eye. In this case, light rays come to a focus behind the retina. The cause of Hyperopia could be that the cornea is too flat (Corneal Hyperopia), or the globe of the eye is too short (Axial Hyperopia).
The far point of the hyperopic eye can vary too. In theory, it is beyond optical infinity/20 feet. Some Hyperopes can see fine in the distance, but cannot see close up. Others need corrective lenses all of the time. Plus (convex) lenses are used for the correction of Hyperopia, to redirect the focus of light so that it reaches the retina. One particular type of Hyperopia, Latent Hyperopia, is difficult to detect without the use of cycloplegics, eye drops to relax the muscle inside of the eye to stop accommodation. Children with Latent Hyperopia over-use their accommodative system, which masks their refractive error. It is crucial that children see the eye doctor prior to entering the first grade, and then again before the third grade. Consult your local eye care provider for more information.
Astigmia is the plural for Astigmatism, of which there are five types:
1. Simple Myopic Astigmatism
2. Simple Hyperopic Astigmatism
3. Compound Myopic Astigmatism
4. Compound Hyperopic Astigmatism
5. Mixed Astigmatism
In each type, light rays do not make a point focus before or beyond the retina, but have two major meridians of focus, usually 90 degrees apart. If you do not have Astigmatism, your eyes are spherical. If you have Astigmatism, your eyes are aspherical. A spherical eye (myopic or hyperopic) requires a spherical lens (concave or convex) to correct its refractive error. An aspherical eye needs a lens that corrects for the error in refraction differently in the major meridians. A way to imagine this is to compare the shapes with a basketball and a football.
If you could measure the curvature in a basketball, it would be the same all the way around, in all meridians. If you could measure curvatures in a football, they would vary…a flatter side and a steeper side. Similarly, the astigmatic eye has some areas with flatter curvature, and some with steeper curves. A simple convex or concave lens will not correct this refractive error…it requires an ophthalmic lens that has two major meridians of curvature, each to offset that of the eye. These lenses are toroidal in nature, and called “cylinder lenses”. The five types of astigmatism are classifications of where the major meridian light rays focus: on/in front of the retina, on/beyond the retina, both in front of the retina, at separate points, both beyond the retina, at separate points, or one in front of the retina, and one behind.
Myopia or Hyperopia can be combined with Astigmatism, in which case a spherocylinder lens is used for its correction. This lens design, known as “cylinder lenses” is commonly used for the correction of Astigmatism.