Understanding the Basics

Refractive Errors

First, let’s discuss some background on how the eye works and define the terms commonly used in the eye care field. Your “refractive error” is the measurement or prescription that tells us how to optically correct your vision. In order to calculate this, we perform a number of tests that measure how light and images pass through your eye. We measure how light refracts or bends, as it passes through your eye’s cornea (outer window of the eye) and lens (inside the eye) to focus on the back surface of your eye (the retina) to form a picture that your brain processes. One way to think of how your eyes work is to think of the inner workings of a camera.

Refractive Error Example

The front surface, or cornea, has certain magnification power just like a camera lens, but your cornea is not always shaped perfectly like a camera lens. When the cornea is not perfectly shaped, the images cannot be clearly projected onto your retina. This causes imperfections or refractive errors that are described below. Once images pass through the cornea, a second interior lens, which is flexible when we are young, also helps to focus the image on the retina.

When we test your vision, we measure the focusing strengths of both the cornea and the natural lens and how they work together so we can determine what we need to do to help you achieve your best possible vision.

Myopia (Nearsightedness)

Nearsightedness occurs when the cornea is too steep or the eye is too long. This means the light that enters the eye focuses in front of the retina causing far and intermediate objects to appear blurry.

Hyperopia (Farsightedness)

Farsightedness occurs when the cornea is too flat or the eye is too short. This causes light to focus past the retina resulting in blurry near or intermediate vision. Those who have high amounts of farsightedness are very dependent on glasses or contacts; however, many people who are only slightly farsighted may not realize focusing problems until they are in their 40s.


Astigmatism is caused by an irregularly shaped cornea or lens and can occur along with nearsightedness or farsightedness. In a perfect eye, the cornea should be uniformly curved in all directions like a bowl, but with astigmatism, it is shaped more like the back of a spoon – with a rounder dimension and a flatter dimension – resulting in a blurred or double vision at all distances.


Presbyopia (a need for reading glasses) Presbyopia occurs when the interior lens of the eye loses its ability to change focusing power simply because of age. As we age, this lens becomes less flexible. On average, around ages 40 to 50, most people gradually lose the ability to see up close when performing activities such as reading a newspaper, book, or menu. You will notice that presbyopia develops progressively so that reading or near vision will continue to worsen and become blurrier over time.

Many people nickname presbyopia “the long-arm disease” because they develop the need to hold reading materials farther and farther away. Eventually, this condition may also affect your intermediate vision, such as working at computers. Our presbyopic patients typically benefit from either presbyopia correction with the Raindrop inlay, blended vision LASIK (one eye focused for near), or replacing the natural lens with a lens implant.

The best choice among these options varies from patient to patient, so we recommend a consultation to discuss how these differences are relevant to your unique eye anatomy.

Slade & Baker Vision patient discusses her LASIK experience from a nurse's perspective

If you have questions, please do not ever hesitate to call our office at 713-626-5544 and speak to our experts and schedule a consultation today!

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Houston, TX 77027

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3900 Essex Lane - Suite 101 - Houston, TX 77027  -  713-626-5544