It is the ability to perceive small details with your eyes.
Visual acuity describes the acuity of vision.
The main measurement tool is the letter presented in 1862 by Donders and Snellen at Eye Infirmary in Utrecht in the Netherlands.
Visual acuity measurement is so common that visual acuity measurement is often mistaken as a single indicator for vision in general.
This is a misconception. Loss of visual acuity can detect many disorders, but not all.
A prime example is glaucoma , which can cause extensive and irreversible visual field damage before visual acuity is affected.
The acuity of the letter graph tells us something about the very small retinal area in which the visible letter is projected.
When the image of that letter is blurred due to optical factors (opacities, refractive error), the surrounding image will be similarly blurred.
But when the loss of visual acuity is due to retinal factors, the acuity of the chart tells us nothing about how the surrounding retina works.
Visual acuity can predict many consequences of vision loss, but not all. It is usually measured in black letters on a uniformly white background.
This condition resembles the reading task, but most other activities of daily living (ADL) involve objects that are much larger, have less contrast, and are presented in a busier context.
Low contrast tests, discussed in the section on Contrast Sensitivity, offer important additional information that high contrast acuity alone cannot provide.
Testing under standard lighting is also not sufficient for conditions that require extra high or very low lighting.
What are visual acuity tests used for?
Visual acuity measurement is a good screening tool because normal visual acuity requires all levels of the visual system to function properly.
The eye’s optical system must project a sharp image of the outside world onto the retina. The retina must be able to translate this image into neuronal impulses.
Finally, neural impulses must travel to the brain, where they are analyzed and recognized. Therefore, a wide range of different visual disorders (but not all) can affect visual acuity.
Because visual acuity is so easily measured, it is used as a primary eligibility criterion, such as 20/20 for a pilot’s license, 20/40 for a driver’s license, 20/200 for certain disability benefits.
However, the ability to multitask depends on many more factors than visual acuity alone.
For those disorders that affect visual acuity, it is also a good follow-up tool to document whether a condition is worsening or recovering or to record the effect of various interventions.
For all these purposes, it is desirable that measurement methods are well standardized and the same from office to office.
This was the main incentive to produce a standardized test for the Early Treatment Diabetic Retinopathy Study (ETDRS).
Finally, letter diagrams are often used as an aid in subjective refraction. This is the least demanding application as the usual question is just: which lens is better, A or B?
How is visual acuity defined and recorded?
Most people are familiar with the notation for visual acuity as a fraction, but few understand what these fractions mean.
However, the explanation is simple. If a subject needs letters that are twice as large or twice as close as a standard eye needs, that person’s visual acuity is said to be 1/2.
If the required letters are five times as large, visual acuity is 1/5, if it is ten times greater – 1/10, etc.
The value of these fractions can be expressed in different ways. E.g. 1/2 = 20/40 = 6/12 = 0.5 or 1/5 = 20/100 = 6/30 = 0.2.
The Recognition Requirement is also known as MAR, so MAR = 1 / V and V = 1 / MAR.
Snellen insisted that the numerator of these fractions should indicate the test distance; eg 20 /… for 20 feet, 5 /… for 5 meters, 6 /… for 6 meters.
Today, this convention is rarely followed. In the United States, the notation 20 /… is used routinely, even if the test distance is not 20 feet.
In Europe, decimal notation is common; in Great Britain (and former British dominions) the notation 6 /… is common.
According to the ICO Visual Acuity Measurement Standard (1984), a line is considered read if “more than half” of the characters are correctly identified.
For an ETDRS table with 5 letters per line, this means 3 or more fixes.
What is the reference standard?
Since the value of the visual acuity fraction compares the performance of a subject with the performance of a standard eye, that standard must be defined.
Snellen chose to define it as the ability to recognize one of its letters when it subtends a visual angle of 5 minutes of arc.
Louise Sloan later coined the name M unit to describe this unit of measurement. An M unit subtend a visual angle of 5 arc min to 1 meter.
Simple geometry tells us that the same visual angle applies for 2 M units at 2 meters, 3 M units at 3 meters, etc.
The 1M unit is also the size of the average newsprint, but that is not the basis of its definition.
The visual acuity notations of 1/1, 4/4, 6/6, 20/20, 1.0 refer to this reference standard.
It’s a common mistake to equate 20/20 with normal (that is, average) or even perfect vision. Snellen chooses his standard because it is “easy to recognize.”
Healthy young adults always exceed the standard; if he had used “average” acuity as a reference standard, half the population would have failed.
“Normal” visual acuity for healthy eyes is one or two lines better than 20/20. In population samples, mean acuity does not drop to the 20/20 level until the 60s or 70s.
Always remember that the 20/20 reference standard does not refer to the average acuity of American eyes.
Like the American standard foot, it is defined independently of the “normal” length of American feet.
What should the measurement steps be?
Since letter graphics only contain discrete font sizes, the accuracy of the measurement depends on the size of the step between lines.
Most traditional US graphics have an irregular sequence of font sizes (just like the original Snellen graphic).
From 20/15 to 20/20 is an increase of 33%, from 20/20 to 20/25 it is 25%, from 20/60 to 20/70 it is 17%, but from 20/100 to 20/200 it is 100%. It was soon recognized that equal step sizes would be desirable.
The first painting with such a sequence was proposed in 1868 by John Green of St. Louis, who had spent time working with Snellen as he toured Europe after his training in ophthalmology.
He proposed a geometric (logarithmic) sequence:
1.0 1.25 1.6 2.0 2.5 3.0 4.0 5.0 6.3 8.0 10.
This sequence later became known as the “Preferred Numbers” series.
Unfortunately, Green was way ahead of his time; his proposal was forgotten and it would take a century for this sequence to be generally accepted for the measurement of visual acuity.
What is the series of Preferred Numbers?
Many different geometric progressions are possible (progressions with a constant relationship between adjacent terms).
The progression used in the ETDRS protocol and in all Precision Vision professional charts is known as the preferred number series.
This series is defined in standard n. 3 of the International Standards Organization and is used in a wide variety of industry standards.
The series of preferred numbers:
It fits well with the decimal system, since each step represents the same proportion 10√10 (100.1).
So 10 steps equals exactly 10x, and after 10 steps, the same digits reappear with only one decimal place shift.
Preferred numbers are convenient because, with only slight rounding, the series contains mostly whole numbers.
Each step represents a 4: 5 ratio (rounding error = + 0.7%), 3 steps are equal to a factor of 2x (rounding error = -0.2%). The same ratios are used to calculate decibels; 3 decibels = 2x.
Being anchored at 1.0 (10, 100, 1000, etc.) the series is suitable for visual acuity, since the reciprocal of a preferred number and the product or quotient of two preferred numbers is again a preferred number
Therefore, if the font sizes (the denominator of the Snellen fraction) and the viewing distances (the numerator of the Snellen fraction) follow this series, so will the resulting visual acuity values.
This works well for 20 /… and decimal notations; in countries using the 6 /… notation, strict adherence to the series would require a test distance of 6.3 m instead of 6 m (6.3 and 3.2 are preferred numbers, 6 and 3 are not).
Since the difference between 6 and 6.3 is only 5% (1/5 of a line, 1 letter ETDRS), this difference can be ignored for ordinary clinical measurements, where the precision of the measurement is on the order of a line.
For research studies where multiple acuity measurements are averaged for greater precision, the difference can be significant.
How can I convert between different notations?
In the United States the notation 20 /… is used commonly used as an equivalent notation, which means that the same notation is used regardless of the test distance.
In Europe, the decimal notation for visual acuity is prominent, while in Britain the 6 / notation prevails… others may wish to use a true Snellen fraction.