March 10, 2015
Is your eye doctor doing their best to ensure you see better than 20/20?
The term “20/20” has been the standard for good vision since the 1800’s. But did you know that many people have the ability to see better than 20/20?
20/20 vision is a term used to express normal visual acuity (the clarity or sharpness of vision) measured at a distance of 20 feet. If you have 20/20 vision, you can see clearly at 20 feet what should normally be seen at that distance. If you have 20/100 vision, it means that you must be as close as 20 feet to see what a person with normal vision can see at 100 feet.
At 20 feet or 6 meters, a human eye with nominal performance is able to separate contours that are approximately 1.75 mm apart. A vision of 20/40 corresponds to lower than nominal performance, a vision of 20/15 to better performance. Some healthy eyes can even distinguish contours as small as 0.875mm apart at 20 feet, which equals 20/10 vision. Many professional baseball players have 20/10 acuity.
20/20 acuity does not necessarily mean perfect vision. 20/20 vision only indicates the sharpness or clarity of vision at a distance. There are other important vision skills, including peripheral awareness or side vision, eye coordination, depth perception, focusing ability and color vision that contribute to your overall visual ability. Another important aspect of vision is contrast sensitivity, or the ability to determine dark gray letters on a light gray background, such as a newspaper. A patient can have 20/20 acuity, but poor contrast sensitivity, resulting in frustrations driving at night or reading a newspaper. A common eye condition that can reduce your contrast sensitivity would be cataracts. A cataract is a yellowing or discoloration of your natural lens inside your eye, and they begin to appear typically in your fifth or sixth decade of life. Some patients who smoke, take steroids, or eat poorly can get cataracts early in life. In rare cases, infants can get a cataract, which is why all children need an eye exam at an early age. Lazy eye can also cause reduced vision, but can be reversed in many instances if diagnosed before age three.
The classic example of an eye chart is the Snellen eye chart, developed by Dutch eye doctor Hermann Snellen in the 1860s. There are many variations of the Snellen eye chart, but in general they show 11 rows of capital letters. The top row contains one letter (usually the “big E,” but other letters can be used). The other rows contain letters that are progressively smaller, and digital charts now allow the letters to be organized in random so patients cannot memorize the chart.
The portion of the eye exam where your eye doctor measures your prescription (“which is better, choice one or two”) is called the refraction. This is where we find out if you are near-sighted, far-sighted or have astigmatism.
So how do the doctors at Vision Center P.C. in Muscatine, Wapello, and Tipton make their refraction unique to make sure patients see to full potential?
1. Optometrists at Vision Center P.C. start with a good foundation. We have a computerized auto-refractor that can measure your eyes in less than 10 seconds to get your spectacle prescription within 97% accuracy. This gives your doctor a starting point to find your prescription for glasses.
2. We analyze your previous prescription and vision through your old glasses to gain insight to your current “refractive” state.
3. We have the patient look through the phoroptor, and run through a series of questions (which is better, one or two), and end with a final prescription. We also ensure that both eyes are balanced and fuse the image together. If done correctly, most healthy eyes will see better than 20/20. If a patient has cataracts, dry eyes, macular degeneration, or a lazy eye, they may not be able to see 20/20, even with the best pair of glasses.
4. Doctors may dilate the eyes before the refraction in some cases to relax the muscles inside the eye, to ensure a correct and accurate prescription. This is very important, especially in young patients, since they can inadvertently focus during the doctor’s measurements and give errors on their refraction.
5. When measuring infants, toddlers, patients who speak a foreign language, or those with a mental handicap, the doctor will use a hand-held light called a retinoscope. When you shine this light into a dilated eye, you can determine the patient’s prescription with very close accuracy without the patient answering any questions.
6. Finally, we measure the muscles that move the eyes, to ensure there is no misalignment that would cause eyestrain. If the eyes have a minor misalignment, patients may see double when they are tired, or get eyestrain after reading or driving. We can place small prisms in your glasses to allow your eyes to fuse together more easily, and to allow your eye muscles to relax. This creates better depth perception and less fatigue at the end of the day.
Patients ask—“If a doctor can measure your spectacle prescription with computerized equipment and a retinoscope, then why do they ask the questions—which is better—one or two? The answer –the doctor is making sure the prescription feels comfortable to the patient’s eyes and is not too strong for them to adapt. If a patient has a large change in their prescription, the doctor may only give them 80-90% of the change. If a patient has an occupation where they are always looking in the distance (truck driver for example) they may tailor the prescription differently than someone who is spending all day in an office or on a computer (accountant or computer programmer).
Our goal at Vision Center P.C. in Muscatine, Wapello, and Tipton is to obtain the correct prescription that allows our patients to see as clear as possible, and minimize fatigue at the end of the day.