What is the Best Corrected Visual Acuity?
The best corrected visual acuity is the highest quality of vision a person can have wearing glasses or contact lenses. It is often abbreviated as BCVA. Oftentimes the best corrected visual acuity is 20/20. However, in some cases, it is reduced due to eye diseases or other conditions that make the vision blurry.
Key Points
- Best corrected visual acuity refers to the best resolution achievable using vision correction, commonly with the help of glasses and contact lenses.
- Best corrected visual acuity is measured in a comprehensive eye exam using a technique called refraction, where a doctor shows two options of lenses and asks the patient which one appears clearer.
- Although many people achieve a best corrected visual acuity of 20/20, the best corrected visual acuity may be reduced due to eye diseases or other eye conditions.
Understanding Best Corrected Visual Acuity
Visual acuity is a measure of the sharpness and clarity of the images formed by the eye. It measures the limit of resolution possible by the retina. The standard visual acuity is 20/20, the smallest size of letters an eye doctor checks for in an exam. It is possible for some people to read smaller letters than 20/20, as small as 20/15 or even 20/10. However, achieving 20/20 vision is usually the best corrected visual acuity an eye doctor tests for.
Visual acuity is measured as a fraction. The top number (numerator) is the testing distance in feet, or what you see. The bottom number (denominator) is the letter size, or what the average person is able to see. For example, 20/20 vision means you see at 20 feet what the average patient also sees at 20 feet. 20/40 vision means you only see at 20 feet what the average person sees at 40 feet.
The standard testing distance to measure visual acuity is 20 feet away. This is why the numerator of “20” in the visual acuity stays constant, while the denominator changes. For example, visual acuity may be measured as 20/25, 20/30, 20/40, up to 20/400. For all visual acuities you’re tested at 20 feet, but the size of the letter you can see changes. The standard 20-foot test distance may be replicated in the exam room using mirrors and projectors, so the eye chart may not be physically twenty feet away.
Normal Functioning Of The Eye
In a normal eye, light reflecting from an object passes through the lens and forms an image on the retina. Rods and cones sense the light stimulation and convert it into neuronal signals. The neuronal signals reach the visual cortex in the brain, where the signals are interpreted.
The lens plays a crucial role in refraction, which refers to a change in the direction of light when passed through a medium. Objects of different sizes and at different distances require the adjustment of the degree of refraction to form a clear image of the object. Clear vision is achieved by changing the shape of the lens and pupil.
Causes of Low Visual Acuity
- Refractive
Myopia (nearsightedness), hyperopia (farsightedness) and astigmatism are refractive conditions that limit visual acuity due to how light does not focus perfectly onto the retina. However, most people are able to have a best corrected visual acuity of 20/20 after having their vision corrected with glasses or contact lenses. - Accommodative
Presbyopia is a condition causing reduced visual acuity when reading, for older aged patients, due to the natural loss of focusing power over time. People with presbyopia are prescribed a reading power in their glasses to assist with magnification of small text. - Cornea
Injury to the cornea, the clear front covering of the eye, can cause permanently reduced best corrected visual acuity that cannot be fixed with glasses. When the cornea gets injured, it scars over and the scarred area is no longer clear to see through. Common causes of corneal injuries are foreign objects entering the eyes, chemical burns, or contact lens infections. - Lens
As the lens gets opacified and yellow over time, cataracts develop. Cataracts cause reduced best corrected visual acuity due to cloudy vision and glare. However, cataracts are removable with surgery and patients tend to have excellent best corrected visual acuity after cataract surgery. - Retina
Diseases of the retina can cause reduced best corrected visual acuity. Examples of retinal conditions that affect the eye are macular degeneration, diabetic retinopathy, retinal detachment, and many others. Retinal conditions affecting the vision are assessed with a dilated eye exam.
Visual Acuity Testing
Visual acuity is tested to predict the prescription of the eyes and to monitor the progress of eye diseases. The progression of eye diseases such as cataracts, glaucoma, and diabetic retinopathy are monitored using visual acuity, along with other tests. If a person with eye disease has reduced vision, eye doctors can determine if the reduced best corrected visual acuity is due to the progression of the underlying disease.
The visual acuity test is performed to determine the sharpness and clarity of vision. Your eye doctor covers one eye with an occluding paddle to test the visual acuity of each eye individually. You’re shown an eye chart with letters, or shapes for younger children, and asked to read a line of letters. You are instructed to read the smallest line you can, in order to measure the threshold, or the limit of your acuity. The best corrected visual acuity is measured as the smallest line you can read with refractive correction such as glasses or contact lenses.
The best corrected visual acuity is determined using a technique called refraction, or monocular subjective refraction. The goal of a refraction is to determine the appropriate lens that allows maximum vision. The patient is shown two options of lenses and is asked to choose which lens is more clear. Using the refraction technique and the patient’s feedback, a glasses or contact lens prescription is finalized. During the refraction, the eyecare provider checks for refractive errors such as nearsightedness, farsightedness, and astigmatism. A refraction is the preferred method of determining the best corrected visual acuity.
