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?
- The Snellen eye chart, with its familiar rows of letters in decreasing sizes, is the most common visual acuity test. While standing or sitting 20 feet from the chart, you are instructed to read each row until you no longer can.
- The Random E test uses a capital letter E that gets smaller in size and also changes direction (up, down, left, right). The test is completed when you can’t tell which direction the E is facing.
- Simplified testing for children. The child is given cards with symbols or letters, which are matched to those on the chart.
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:
- a pediatrician or other doctor
- a technician or medical assistant
- a nurse
- an optician
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:
- best corrected (with glasses or contact lenses) visual acuity
- peripheral (side) vision
- depth perception (seeing objects in three dimensions)
- eye movement
- binocular (two-eye) function
- the health of the retina, in the back of your eye
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:Janet Marsden
Nurse Advisor: Community Eye Health Journal, London, UK.
Sue Stevens
Nurse Advisor (retired): Community Eye Health Journal.
Anne Ebri
West Africa Sub-Regional Manager: Brien Holden Vision Institute, Calabar, Nigeria.
− 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.
- After testing and recording the VA for the right eye repeat now for the left eye.
- If the patient wears spectacles for distance vision, now test the VA in each eye with the spectacles on.
- If the visual acuity in either eye is less than 6/6, one can measure the visual acuity with a pin hole (see panel).
- The VA is recorded for each eye in the patient’s notes.
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 test
Using 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
- Position the patient 6 metres from the chart.
- Ask the patient to cover one eye with the occluder.
- Position the pin hole over the eye to be tested so they can see the chart through the pinhole.
- Test one eye at a time by following the same procedure used to test visual acuity.
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.
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