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What is visual acuity?Visual acuity measures how sharp your vision is at a distance. It is usually tested by reading an eye chart. How is visual acuity tested?
You can also test visual acuity at home. But home tests are not meant to replace an exam given by an eye care professional. Your ophthalmologist tests visual acuity as part of a comprehensive eye exam, but others may perform the test, including:
Visual acuity can also be tested at schools and most states require testing before issuing a driver’s license. What do my test results mean?The results of a visual acuity test are written as a fraction in the U.S. The top number is always the same and signifies the distance between you and the chart, 20 feet. The bottom number is the last line read correctly. For example, a result of “20/20” —normal visual acuity—means you read the line that those with normal vision can read. Visual acuity decreases as the bottom number gets larger. A result of 20/40 means you can see at 20 feet what those with normal vision can see from 40 feet away. Why would I not have 20/20 vision?Visual acuity less than 20/20 is common. In most cases, blurry vision from a refractive error (nearsightedness, farsightedness or astigmatism) is to blame. The good news is that glasses, contact lenses or surgery can likely improve your vision to 20/20 or close to it. Sometimes eye disease, infection or other health problems reduce your visual acuity. In this case, you and your doctor can discuss next steps and what treatment may be needed. Visual acuity isn’t everything…We all want uncorrected 20/20 vision, but remember, visual acuity is just one facet of your eye health. With regular eye exams, your ophthalmologist can monitor the entire range of your visual function, including:
Comm Eye Health Vol. 32 No. 107 2019 pp 46. Published online 17 December 2019. Visual
acuity is a measure of the ability of the eye to distinguish the details of objects. Visual acuity testing is part of every eye examination. It is important that it is done well, and accurately, as an incorrect measurement can lead to inappropriate decisions and management. It is important to assess visual acuity in a consistent way in order to detect any changes in vision. One eye is tested at a time. Equipment
Preparation
Testing and recording visual acuity
If the patient cannot read the largest (top) letter at 6 metres, either: − move them closer to the chart, 1 metre at a time, until the top letter can be seen – the VA will then be recorded as 5/60 or 4/60, etc. or − hold up your fingers at varying distances (5 metres, 4 metres etc. and record the vision as counting fingers (CF) at the maximum distance they can see between 5 and 1 metre, i.e. VA = CF 5m or VA = CF 1m. If the patient cannot count fingers at 1 metre, wave your hand and check if he/she can see this. This is recorded as hand movements (HM): VA = HM. If the patient cannot see hand movements, shine a torch in the eye and ask if they can see the light. If they can, record ‘perception of light’: VA = PL. If they cannot see the light, record ‘no perception of light’: VA = NPL.
For example: Right VA = 6/18 without spectacles Right VA = 6/6 with spectacles Left VA = HM without spectacles Left VA = HM with spectacles The pinhole testUsing a pinhole reduces the need to focus the light that enters the eye, and people with a refractive error, such as myopia, can usually see better with the pinhole than without it. Steps
If the person can read more letters with the pinhole than without, they are likely to have a refractive error, such as myopia. All patients (adults and children) whose acuity improves with a pinhole should undergo a full refraction to see whether they require spectacles, and of what power. Adapted from: Marsden J, Stevens S, Ebri A. How to measure distance visual acuity. Comm Eye Health J 2014;27(85):16. Hennely ML. How to detect myopia in the eye clinic. Comm Eye Health J 2019;32(105):15-16. CC BY-SA 4.0
At what distance should a patient read a Snellen chart?The visual acuity test is used to determine the smallest letters you can read on a standardized chart (Snellen chart) or a card held 20 feet (6 meters) away.
How far should you stand from a vision test?Stand 10 feet (3 meters) away from the chart. If you wear glasses or contact lenses for distance vision, wear them for the test. Check each eye separately, first the right and then the left. Keep both eyes open and cover one eye with the palm of the hand.
What is 20 40 vision on eye chart?If you have 20/40 vision, it means that you need to be as close as 20 feet to see what a person with normal vision can see at 40 feet. The largest letter on an eye chart often represents an acuity of 20/200 which is associated with the term "legally blind." You will be asked to read the letters one eye at a time.
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