Eye Examination and Prescription for Myopia

By Dr. Noppadol Srisuratnamethakul, Doctor of Optometry

If you feel your vision is blurred, often have to strain your eyes, or start getting headaches, it might be time for an eye exam. An eye exam is not just about reading letters from a test chart; there are many steps involved to find detailed information about your vision problems. Let's look at what an eye exam is and why it's important. 

What is an Eye Exam and Why is it Important?

An eye exam is a check of your vision ability, both near and far, to assess your prescription and find any abnormalities, such as nearsightedness, farsightedness, or astigmatism.

Having an eye exam is important because it helps us identify eye problems early and correct them appropriately, for example, by wearing eyeglasses, contact lenses, or having LASIK surgery. Furthermore, regular eye exams can help detect eye diseases in the early stages, which increases the chance of a successful cure. 

Popular Methods for Measuring Eye Prescription

Popular methods for eye exams include : 

1. Visual Acuity (VA)

   is an assessment of how clearly the eyes can see, both near and far, using a Snellen Chart which consists of letters or numbers that progressively decrease in size.

   The patient must stand or sit at a distance from the chart and read the letters or numbers, starting from the top row with the largest characters and gradually moving down one row at a time. During the test, one eye will be covered, alternating between sides. If unsure or unable to see clearly, the patient may guess. The official will then record the reading results to assess the level of vision.

2. The Refraction Test)

   is used to identify abnormal conditions such as nearsightedness, farsightedness, and astigmatism, which is important information for ordering eyeglasses or selecting contact lenses. It can be measured by reading number and letter charts through a device called a Phoropter.

   The Phoropter machine has two circular slots similar to eyeglasses mounted on it. The patient must sit in the correct position so their eyes can look through the lenses of the machine. The official will then adjust the lenses on the machine to be close to the patient's original prescription and ask them to try reading an image. If the image is still blurry or unclear, the official will adjust the lenses again until the patient can read clearly. When the examination is complete, the official will record the lens power used by the patient, which is the prescription value indicating the abnormality.

Detailed Steps of an Eye Exam

Ever wondered what they do at an optical shop when you go for an eye exam? The detailed steps of an eye exam are as follows : 

1. Determine the Distance and Size of the Target

   When measuring the vision of a patient with an eye problem, the first thing to consider is the distance used for the eye exam. The ideal measuring distance should be about 6 meters or 20 feet, measured from the patient's eye to the target image they are looking at. If the measuring distance is less than 6 meters, the resulting value will be higher than the actual prescription because the patient's focusing mechanism (accommodation) will automatically engage as the distance is closer. Furthermore, the size of the image used for the measurement must be appropriate for the distance, as the charts we commonly use come in two sizes: a chart for a 3-meter distance and a chart for a 6-meter distance. If we use the wrong chart for the distance, the resulting eye prescription value will also be incorrect. 

   

2. Measure VA

   After we have determined the distance and size of the image, the first thing we must measure is VA (Visual Acuity), which is the ability to see. We will start by measuring one eye at a time, having the patient use an occluder to cover the eye not being measured without closing it. After measuring each eye individually, we then measure with both eyes open.

   If the patient has old eyeglasses, VA must be measured twice: the first time without glasses (unaided VA), and the second time with the patient wearing their old glasses, to know if the old glasses are still providing good vision. For a normal person, the VA result should be 20/20 or 6/6. 

   

3. Test with Pinhole

   For people with eye problems, the VA value will not be within the normal range. We must use a pinhole for testing to see if the eye problem can be corrected with eyeglasses. If the patient looks through the pinhole and the VA value improves, it indicates that the eye problem can be corrected with eyeglasses. However, if the patient looks through the pinhole and the VA value remains the same or worsens, it indicates that the eye problem cannot be corrected with eyeglasses, and the patient should be advised to see an ophthalmologist to find the cause of the blurred vision, as it might be due to an underlying eye health issue or eye disease, so that the problem can be addressed directly. 

   

4. Measure Vision using Objective and Subjective Methods

   Once we have completed the VA measurement, we will start measuring the eye prescription using the Objective method, which is where we find the initial prescription value without the patient having to answer anything. The official will determine the prescription value themselves, using methods such as Retinoscopy or Autorefraction. After we get the initial prescription value, we will proceed to find the most suitable prescription for the patient using the Subjective method, where the patient must cooperate by answering questions, which is divided into the following sub-steps :

   1. Find the best short-sightedness or long-sightedness value before finding the astigmatism value using the Red-Green Test or Step Down method. 

      

   2. Find the most suitable astigmatism value and axis using the Cross Cylinder.

      
   3. Find the best short-sightedness or long-sightedness value again using the Red-Green Test or Step Down method to get the prescription that provides the best vision for each eye, while controlling for over-correction. All these steps are performed on one eye at a time, resulting in the best prescription for each eye, and then we proceed to the next eye measurement step.
   4. Find the most suitable prescription for using both eyes together. This step is crucial and cannot be skipped, as measuring one eye at a time by covering the other may lead to different levels of accommodation in each eye. Therefore, we must find the most suitable prescription that will provide the best vision for both eyes and make both eyes see as similarly as possible, by using the Step Down method with both eyes together, but using the occluder to alternate covering the eyes so the patient can compare the sharpness of each eye.

   5. Once the most suitable prescription is found, the patient will be given trial glasses to walk around or use their vision normally to check if there are any comfort issues, or if wearing them causes headaches or eye strain. 

      

How Many Types of Eye Prescriptions Are There?

Eye prescription is a value that indicates the ability to see. The prescriptions obtained from an eye exam are divided into 3 types :

1. Nearsightedness (Myopia)

Nearsightedness means that when an image of a distant object passes through the eye, the image focuses before the retina, causing the image falling on the retina to be blurry, resulting in blurred distance vision. However, if we move the object closer, the focal point of the image will move backward. When the image is moved close enough that the focal point falls exactly on the retina, we will see the image clearly. 

Levels of Nearsightedness

• Normal Nearsightedness : The prescription value is in the range of -0.25 to -3.00 Diopters (less than 300). People with this level of nearsightedness see distant objects unclearly but have no risk of complications. It can be corrected by wearing eyeglasses, contact lenses, or other treatments.
• High Nearsightedness : The prescription value is greater than -6.00 Diopters (greater than 600), which carries a high risk of vision complications such as cataracts, glaucoma, retinal detachment, macular degeneration, or abnormal blood vessel growth in the retina.

2. Farsightedness (Hyperopia)

Farsightedness is a condition where near objects are seen unclearly, but distant objects can be seen normally. It is caused by an abnormally short eyeball or a cornea or lens with too little curvature, which causes light entering the eye to focus at a point behind the retina, resulting in a blurry image on the retina, especially for near objects. 

Levels of Farsightedness

• Mild Farsightedness: The prescription value is approximately +1.00 to +2.00 Diopters.
• Moderate Farsightedness: The prescription value is approximately +2.25 to +5.00 Diopters.
• High Farsightedness: The prescription value is greater than +5.00 Diopters and above.

3. Astigmatism (Astigmatism)

Astigmatism is a condition where vision is unclear both near and far. It is caused by an abnormality in the cornea or lens where the curvature is not uniform in all directions, causing light entering the eye to focus at multiple points on the retina, resulting in a distorted and non-sharp image on the retina. 

Types of Astigmatism

• Regular Astigmatism: More common, caused by the cornea or lens having a non-uniform curvature in one direction, such as vertical, horizontal, or oblique, causing the light entering the eye not to focus at a single point, but often focusing as a line or ellipse.
• Irregular Astigmatism: Caused by the cornea or lens having a non-uniform curvature in multiple directions, resulting in distorted and unclear vision. It is often caused by eye injury and cannot be treated by wearing eyeglasses.

How to Correct Nearsightedness

• Wear eyeglasses or contact lenses with the appropriate prescription.
• Have LASIK surgery, which uses a laser to correct abnormal vision prescriptions, including nearsightedness, farsightedness, and astigmatism.

Ways to Prevent Vision Problems and Care for Eye Health

• Avoid staring at computer or mobile phone screens for long periods. You should rest your eyes.
• Adjust the lighting appropriately when reading or working.
• Eat foods beneficial for the eyes, such as green leafy vegetables, carrots, and fish containing Omega-3.
• Drink enough water to prevent dry eyes.
• Have regular annual eye exams to prevent potential eye problems.

Frequently Asked Questions about Eye Exams

1. What should a normal eye prescription value be?

   A normal eye prescription value is 0.00 D (Diopter), which means there is no nearsightedness, farsightedness, or astigmatism.

2. What is 20/200 vision?

   The number 20 in front of the 20/200 vision value refers to a distance of 20 feet, which is the distance at which a person with abnormal vision can read standard-sized letters. The number 200 at the back refers to a distance of 200 feet, which is the distance at which a person with normal vision can read the same set of letters.

   This means that if your vision is 20/200, your vision is 10 times worse than a normal person. Simply put, that person has to be 20 feet away from an object to see it as clearly as a person with normal vision sees it from 200 feet.

3. Is -3.00 Diopters of nearsightedness a lot?

   -3.00 Diopters (or -3.00 D) of nearsightedness is considered moderate level, where distant objects are seen unclearly, and the patient must primarily wear eyeglasses or contact lenses.

4. How to Prepare Before an Eye Exam?
   • Get sufficient eye rest.
   • Remove contact lenses 1-2 hours before the eye exam to allow the cornea to fully recover its shape.
   • Report any information about visual symptoms, such as eye pain, dry eyes, or blurred vision.
   • Bring the eyeglasses or contact lenses you currently use for comparison of prescription values.

Consult with an Eye Care Specialist

For those who need an eye exam and vision correction, you can come and consult your problems with experts and optometrists at ISOPTIK Progressive Eyeglasses Center. We have a team of expert optometrists and ophthalmologists ready to provide consultation, design, and thoroughly analyze your vision use, with Master Bobi closely supervising every step, so you receive comfortable progressive eyeglasses that meet your needs and help elevate your quality of life, along with a maximum satisfaction guarantee of 180 days. 

Free consultation with Grandmaster Bobi, a World Class Expert in 3D Bespoke Ultra Individual Progressive Lenses. Call:  081-538-4200

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