As a practicing optometrist, Dr. W. answers thousands of patient questions a year. Many questions are in regards to eye examinations and what eye doctors are really looking for. Over the next few days Dr. W. will explain what goes on in the eye exam. In Part 1 he explains those 20/20 numbers.
“My eye doctor told me that my vision is 20/400. Am I blind?”
Patients are often confused with the 20/20 number reported by their eye doctor. What does it mean? How is it used? Can it change? It that bad? These are common questions. Let me explain it here just as I do in the exam room.
What is visual acuity?
Visual acuity tells the doctor how well you are able to see at a given distance. It is usually represented in the United States by a 20/X ratio.
How is visual acuity measured?
It is usually obtained by having a patient read the lowest letters possible on a distance chart or up close. For pediatric and illiterate patients we often use numbers or symbols to obtain visual acuity. Acuity can even be measured in infants by presenting a forced choice between striped paddles. Visual acuity can be obtained with or without spectacle or contact lens correction.
How do I make sense of the number ratio?
The top number 20 always stays the same. It means a test distance of 20 feet. The number on the bottom is your score, also in feet. So-called “perfect vision” is designated as 20/20. If your visual acuity was measured as 20/40 it means that you see at 20 feet what somebody with perfect vision can see at 40 feet. Several examples are posted below.
- 20/80. You see at 20 feet what somebody with perfect vision can see at 80 feet. That is, you need to come four times closer than somebody with perfect vision to read the letters.
- 20/400. You see at 20 feet what somebody with perfect vision can see at 400 feet. You must be 20 times closer to make out the letters.
- 20/15. You see at 20 feet what somebody with perfect vision can see at 15 feet. Your vision is, in this case, “better than perfect.”
Is it a bad thing that my visual acuity is poor?
It depends. Unaided acuities (acuity obtained without spectacles or contact lenses) can be pretty bad with just a small amount of prescription. Most folks who wear glasses or contacts will measure in the 20/200 or worse range when they aren’t wearing correction. I usually don’t get to concerned with poor unaided visual acuity. The important number is acuity obtained with correction. 20/20 is always my goal when I determine a patient’s prescription. Everybody should be able to demonstrate 20/20 vision with accurate correction unless there is a medical or physiological condition.
Can my acuity change over time?
Absolutely. Visual acuity is quite poor at birth and improves during the first years of life. If you are nearsighted or farsighted your acuity will deteriorate over the years. Nearsighted (myopic) patients often notice blurry vision in grade school (though it’s onset can be earlier or later.) This blurriness indicates a poor visual acuity. Vision usually stabilizes during the 20’s and 30’s. Near visual acuity often worsens in a patients 40’s. Worsening of visual acuity is often seen as we mature through life due to eye diseases and other conditions.
What conditions can affect my best corrected acuity?
Many eye conditions have the potential to alter acuity to the point that 20/20 vision is impossible even with glasses or contact lenses. The most common are: diabetes, cataract, glaucoma, macular degeneration, lazy eye, trauma to the eye, and infections of the eye.
My goal as a doctor is to provide the best possible vision to everybody. I want to empower my patients with knowledge and understanding regarding their eyes. Visual acuity is just one of many important measurements obtained in the eye exam. Stay tuned for further articles explaining what happens during the eye exam.