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What is Visual Acuity?

By A. B. Kelsey
Updated Mar 03, 2024
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Visual acuity is defined as the clarity or sharpness of vision, which is the ability of the eye to see and distinguish fine details. This is an important factor for a variety of everyday tasks, including reading text, recognizing symbols, and performing assembly work. Good visual acuity is very important when driving, because it helps people recognize landmarks, avoid obstacles, and read road signs. An ophthalmologist or optometrist measures how clearly a person sees during a routine eye exam using a wall chart with symbols or letters. It is determined by the smallest line the patient can read on the chart.

This measurement is typically given as a fraction, such as “20-20” or “20-40.” The first number refers to the distance at which the patient’s vision was tested, which is generally 20 feet (6 meters). The second number indicates the distance from which a normal eye can see the symbol or letter on the chart. If a person’s visual acuity is measured at 20/40, therefore, the person can see at 20 feet what a person with normal vision can see at a distance of 40 feet (12.1 m). The fraction 20/20 is considered normal vision. A measurement of 20/200 or worse is considered legally blind.

A person’s visual acuity can be influenced by several factors, including color, brightness, and contrast. A bright light or color can make it more difficult for the eye to perceive a certain object. On the other hand, the more contrast between an object and its background, the easier it is for the eye to pick out small details. Eyesight is also affected by conditions such as shortsightedness and long sightedness.

Visual acuity is measured for a variety of reasons. An eye doctor decides on a patient’s prescription by determining which corrective lens power will permit the best clarity. Changes in eyesight can be indications of a serious eye disease. Good visual acuity is required for employment in certain occupations, such as airline pilot or police officer. In addition, many jurisdictions require people to have a corrected eyesight of 20/40 in at least one eye before they can receive a driver’s license.

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Discussion Comments
By anon322801 — On Mar 01, 2013

UK. Visual Acuity Clinical Findings

Right: 0.20, Pinhole right: 0.10

Left: 0.10

Can you tell me what this means please? I am having difficulty understanding.

By LisaLou — On Nov 28, 2012

I had laser surgery on my eyes to correct my vision, hoping I wouldn't have to rely on glasses or contact lenses again. Sadly, after a few years, my vision started to regress and now I am almost back to where I was before I had the surgery.

This happened gradually, but I could tell my vision was not as clear as it should be. Every time I went to get my eyes checked, the bottom numbers or letters on the eye chart were a little bit harder to read.

I can still pass the eye exam to get my driver's license without any correction, but this kind of scares me. I don't feel comfortable driving without wearing glasses now because I can't see the signs very well.

Just because the law says I can drive without glasses or contacts, I don't feel like my visual acuity is sharp enough to do so.

By julies — On Nov 27, 2012

@myharley -- I have much better visual acuity with my soft contact lenses when I keep up with cleaning them on a daily basis. If you wear the daily lenses you don't have to worry about this so much. I wear monthly contact lenses and notice if I get lazy and don't clean them, my visual acuity is not nearly as good.

By myharley — On Nov 26, 2012

I have worn contact lenses for over 20 years and notice a difference in my visual acuity with the type of lenses I am wearing. I have always had clearer, sharper vision when I wear hard contact lenses. These aren't as comfortable on my eyes though, so I have switched to the soft lenses.

I can wear the soft contact lenses for longer periods of time, and my eyes feel better, but I still don't feel like I can see as well with them. For me it's a trade off, and I would rather wear my contact lenses all day long without irritation, instead of wearing lenses that aren't as comfortable but give me sharper vision.

By bagley79 — On Nov 26, 2012

Once when my dad and I were taking a trip, we started testing our visual acuity by reading road signs. We would pick out a sign ahead of us and see who could read the sign first.

I was able to read every sign quicker than my dad, and this concerned him. Come to find out, he hadn't had his eyes checked for about four years, and he was due for an eye exam.

When he went to the eye doctor, his prescription had changed. When it comes to visual acuity, it can happen so slowly and gradually that it can be hard for someone to recognize that changes are happening.

By Myrtonos — On Apr 03, 2011

Actually, I think that would be legally considered monocular vision.

By Renegade — On Feb 26, 2011

Birds have immensely effective vision which enables them to have a higher visual acuity than humans do. This is because they depend on their eyesight for survival and finding prey. The vision of an owl requires it to be able to see mice in the dark from a height.

By Armas1313 — On Feb 23, 2011

@anon116336

You would be considered mildly legally blind in the eye which has 20/400 vision. Anything lower than 20/200 vision is considered legal blindness.

By anon116336 — On Oct 06, 2010

if I have 20/20 vision in one eye and 20/400 in the other. by law, is this considered blind.

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