Eye Health Exams & Procedures What Does 20-20 Vision Mean and How Is It Measured? By Troy Bedinghaus, OD Troy Bedinghaus, OD LinkedIn Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association. Learn about our editorial process Updated on November 21, 2022 Medically reviewed by Dagny Zhu, MD Medically reviewed by Dagny Zhu, MD Facebook LinkedIn Twitter Dagny Zhu, MD, is an award-winning opthalmologist and Owner and Medical Director of Hyperspeed LASIK. She specializes in laser vision correction and cataract surgery. Learn about our Medical Expert Board Fact checked by Angela Underwood Fact checked by Angela Underwood LinkedIn Angela Underwood's extensive local, state, and federal healthcare and environmental news coverage includes 911 first-responder compensation policy to the Ciba-Geigy water contamination case in Toms River, NJ. Her additional health-related coverage includes death and dying, skin care, and autism spectrum disorder. Learn about our editorial process Print Healthcare providers and eye doctors tend to throw around the term "20/20" quite a bit, and because of this, most have come to understand that having 20/20 vision is adequate, normal, or at least a good thing. Even the news program "20/20" promises a clearer view of specific stories. PhotoAlto/Eric Audras / Getty Images Measuring Vision Human vision is complicated. It is multifaceted, and eye care professionals need some way to quantify or measure vision for patients. For example, instead of diagnosing "vision as okay," it would be much more accurate to record someone's vision as 20/30. This way, we could track how our vision may or may not change over a certain amount of time. Snellen System of Visual Acuity To many people's surprise, an eye examination in its entirety is quite complicated. Because your eye and vision is part your brain and nervous system, there are many things on your healthcare provider's check-off list. The first and maybe the most important measurement is our central vision, or central visual acuity. This is the part of our vision we use when we aim our eye right at something to view it. In the United States, we use the Snellen system of visual acuity. Named after Herman Snellen, an opthalmologist from the Netherlands, in 1862, the system is based on a series of letters and numbers. The top number, 20, refers to a standard testing distance in feet. Scientists decided on 20 feet because anything we are viewing that is 20 feet or farther is considered optical infinity. They came to that conclusion because of the way light waves travel and the way in which our eye focuses on objects. In a normal, optically perfect human eye, the focusing muscle is in a totally relaxed state when viewing objects at 20 feet or farther. When things begin to move closer to our eyes than 20 feet, our eye begins to change its focus in order to keep them clear. Also, the Snellen system assumes that a normal eye has good acuity if it can resolve certain detail in a letter at 20 feet. Those details are described as the distance at which each element of a letter, for example, a letter E, has an angular height of one minute of arc. One minute of arc is equal to 1/60th of a degree. The entire letter makes up 5 minutes of arc. To understand this, you have to go back to geometry and draw out a triangle and measure the angle of the triangle. The large end of the triangle is a letter E, with five elements...the top bar of the E, a space, the middle bar, a space and the bottom bar of the eye. The angle is five minutes of arc for the whole letter and one minute of arc for each bar. So What Does 20/400 Mean? 20/400 vision is essentially when you can only see the big E (the 20/400 line) at the top of the eye chart. All the lines below that are indistinguishable. In contrast, someone with 20/20 vision, can see all the way down to the 20/20 line. Another way to say this is that someone with 20/400 vision can only distinguish objects placed at 20 feet, while someone with 20/20 vision can distinguish the same object placed 400 feet away. Another way to think about this is to say that if someone is measured to have 20/50 vision, then that person has to move up to 20 feet to be able to resolve the smallest detail in a letter, whereas their completely normal friend can stand way back at 50 feet and resolve the smallest detail. Using Mirrors in Exam Rooms If you think about it, most of the exam rooms optometrists use are not 20 feet long. Interestingly, simply putting up a mirror simulates the 20-foot long testing distance quite well. If a room is ten feet long, putting up a mirror to project the eye chart makes the room appear 20 feet long to the eye. Optometrists have gotten very good at calibrating eye charts according to the exact distance from the patient’s eye to the mirror and then from the mirror to the eye chart projector. With the advent of computerized eye charts, the calibration is even easier. Is 20/20 Really Normal? We have to determine 20/20 vision to be the average normal vision. However, as with most measurements in health care, there is a normal “range” of vision. Some of us may see slightly less than 20/20, say 20/25, and some of us may see better than 20/20, say 20/15, and still be considered normal. The resolution of our central vision is similar to a high-resolution computer monitor. If the pixels are very fine and close together, the resolution of the monitor is better. TV manufacturers actually study visual acuity. There becomes a point where a certain resolution of a TV that is higher than the best human visual acuity will not make a difference. If the resolution of a TV is better than what the human eye can resolve, what is the point? 11 Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. John Hopkins Medicine. Brain Anatomy and How the Brain Works. MyHealth.Alberta.ca. Central Vision. Tsui E, Patel P. Calculated Decisions: Visual acuity testing (Snellen chart). Emergency Medicine Practice. 2020 Apr 1;22(4):CD1-CD2. PMID: 32259420. van Gijn J, Gijselhart JP. [Snellen and his optotypes]. Ned Tijdschr Geneeskd. 2012;156(16):A4416. PMID. 22510417. Vision and Eye Health. Visual Acuity. Britannica. Snellen chart. Britannica. Snellen chart. Bahkir F. Decoding the snellen’s chart. DJO. 2020;30(4). doi. 10.7869/djo.552. Stack Exchange. Can I simulate distance using a series of mirrors? The ray diagrams of this is driving me crazy. TFT Central. Visual Acuity - The Sense and Non-sense of Ultra High Definition Displays. Tech Hive. 8K vs 4K TVs: Double-blind study by Warner Bros. et al reveals most consumers can't tell the difference. By Troy Bedinghaus, OD Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit