acuity often is referred to as “Snellen” acuity. The chart and
the letters are named for a 19th-century Dutch ophthalmologist
Hermann Snellen (1834–1908) who created them as a test of visual
One’s visual acuity is an indication of the clarity or
clearness of one’s vision. It is a measurement of how well a
person sees. The word “acuity” comes from the Latin
acuitas, which means sharpness.
20/20 or 6/6 visual acuity
The reason that the number “20” is used in visual acuity
measurements is because, in the United States, the standard
length of an eye exam room (that is, the distance from the
patient to the acuity chart) is about 20 feet.
In Great Britain, where meters are used instead of feet, a
typical eye exam room is about 6 meters long. Six meters is
19.685 feet, which is close to 20 feet, and usually is
considered to be “close enough” to
infinity. Therefore, instead of using 20/20 for normal
vision, a notation of 6/6 is used in Britain.
Someone with 20/20 or 6/6 vision (visual acuity) is just able
to decipher a letter that subtends a visual angle of 5
minutes of arc (written 5') at the eye.
(5' of arc is 5/60 of a degree, because there are 60' of arc in
1 degree.) What this means is that if you draw a line from the
top of a 20/20 letter to the eye and another line from the
bottom of the letter to the eye, the size of the angle at the
intersection of these two lines at the eye is 5' of arc.
Also, the individual parts of the letter subtend a visual
angle of 1' of arc at the eye. It does not matter how far away
something is from the eye; if it subtends an angle of 5' of arc
at the eye, then a person with 20/20 visual acuity will just be
able to distinguish what it is.
A person with 20/20 vision could stand 30 feet away from a
test chart and just decipher a 20/30 letter on the chart, since
at that distance a 20/30 letter would subtend an angle of 5' of
arc at the person’s eye. That same person could stand 80 feet
away from the chart and be able to decipher a 20/80 letter, or
200 feet away to be able to decipher a 20/200 letter.
20/20 compared with other acuities
Someone with 20/20 visual acuity does not have “perfect”
vision, since it is quite possible to see better than 20/20.
The less the bottom number in the visual acuity ratio, the
better the acuity; and the greater the bottom number, the worse
the acuity. Therefore, 20/15 acuity is better than 20/20
acuity, and 20/30 acuity is worse than 20/20 acuity. Also,
20/15 acuity is equivalent to 6/4.5 acuity, while 20/30 acuity
is the same as 6/9 acuity.
As noted before, although 20/20 is "normal" visual acuity for
most people, it is possible (and, in fact, very common) to be
able to see better than that. For instance, many people have
20/15 visual acuity. A person with 20/15 acuity can stand 20
feet away from an object and see it as well as a person with
20/20 acuity moving up to 15 feet away from the object to view
If that is true, let’s take a person with 20/15 vision
looking at an object from 100 feet away. Where would a person
with 20/20 vision need to stand to see the object just as well?
The answer is 75 feet away from the object. (That is, 15/20 ×
100 feet = 75 feet.)
It even is possible, although not too common, for someone to
have 20/10 visual acuity. Let’s say a person with 20/20 vision
can just detect a ship which is 25 miles away out on the ocean.
A person with 20/10 acuity could be 50 miles away from the ship
and still be able to just detect it. That is, if a person with
20/10 acuity can just tell what an object is, a person with
20/20 vision would need to stand half that distance away to be
able to see what it is.
You can use the same rationale when considering someone with
less than 20/20 acuity. Consider a person with 20/40 visual
acuity (which is what someone needs in most states to acquire a
driver’s license). If a person with 20/20 acuity can just read
a sign which is 60 feet down the road, the person with 20/40
acuity would have to be 30 feet away to read the same sign.
Also, a person with 20/15 acuity could be 80 feet away, and a
person with 20/10 acuity 120 feet away, to read the same sign.
Compared to a person with 20/20 vision reading a sign
30 feet away, how far do people with various visual
acuities need to stand away from the sign to be able to read it
as well as the person with 20/20 acuity? See the following
near visual acuity
Besides a person’s visual acuity being tested at a far
distance, one’s near acuity also can be tested. Testing
typically is done by holding a nearpoint Snellen acuity card at
40 centimeters (about 16 inches). Just as on a far acuity
chart, a 20/20 letter on a near chart subtends a visual angle at
the eye of 5' of arc (5 minutes of arc, or 1/12 of a degree).
Without a lens correction, a myopic (nearsighted) person
generally will have better visual acuity at near than at far,
while a hyperopic (farsighted) person generally will have better
acuity at far than at near. Until the early to mid-forties, a
person with 20/20 distance acuity usually also has 20/20 acuity
at near. However, once
presbyopia sets in, one’s uncorrected near visual acuity
decreases, creating the need for reading glasses or bifocals.
size of a 20/20 letter
When an eye doctor sets up an examination room, care should
be taken in calibrating the size of the letters on the visual
acuity chart (which usually is projected onto a highly
The correct size of a 20/20 letter can be calculated using
the diagram below, where
As an example, let’s say that the viewing distance,
d, is 20 feet.
- the letter’s visual angle subtended at the eye is 5' of
arc (5 minutes of arc), one-half of which is
2.5' of arc,
- d is the distance
(or virtual distance, if using a mirror), along the line of
sight, from the eye to the chart in feet, and
- h is one-half the
height of the 20/20 letter in millimeters.
Since a right angle is formed by the line of sight and the
plane of the acuity chart, then simple trigonometry can be used:
- 2.5' of arc ÷ 60 =
- tangent 0.04167° = h ÷
d = h
÷ 20 feet
- 0.0007272 = h ÷ 6,096
- h = 0.0007272 × 6,096
- h = 4.433 millimeters
- 2h = total height of a
20/20 letter at 20 feet = 8.87
In general, to find the size of a 20/20 letter (in
millimeters), multiply .4433 by
d (where d is the
viewing distance in feet). That is:
.4433 mm/ft ×
d ft = height of 20/20 letter in mm.
When an eye is looking at a far away distance (such as at the
horizon or at the moon or at a star). The rays of light
entering the eye are virtually parallel, and the
crystalline lens of the eye is thin and
relaxed because, essentially, there is zero accommodation.
When an optometrist or an ophthalmologist examines and
performs a refraction on someone’s eyes, it is optimal for the
object being viewed (presumably an acuity chart) to be as far
away as possible from the patient. This is so that the incoming
rays of light are as close to parallel as possible, and the
amount of accommodation (increased curvature) of the crystalline
lens of the eye will be negligible.
Due to space limitations, though, this viewing distance (= “d”
in the diagram above) can be only a few meters away from the
patient in an examination room. Therefore, the goal of an eye
doctor should be to position the eye chart at “optical
infinity,” or the least distance at which there is no
significant accommodation by the crystalline lenses of a
Traditionally, optical infinity has been accepted to be 20
feet or, approximately, 6 meters. However, at this distance,
there is an accommodative demand on the eye of about 1/6 D
(one-sixth of a diopter), which can be significant. For many
people (such as myself), an accommodative fluctuation during an
eye examination of more than 1/8 D can result in a
variable endpoint in measuring a person’s refractive error, and
1/6 D is even greater than 1/8 D.
As a result, it is recommended that the viewing distance (d)
in an examination room should be long enough to create no more
than a 1/8 D accommodative demand on any patient’s
eyes. I maintain, then, that optical infinity, for purposes of
examining the refractive error of the human eye, is at least
8 meters or 26¼ feet, rather than
merely 6 meters or 20 feet.
If lack of space is a problem, front-surface reflective
mirrors usually can be utilized to increase the virtual viewing
distance in an exam room. From the previous section, it can be
seen that the height of a 20/20 letter on an acuity chart,
located at a viewing distance from a patient’s eyes of
d = 26¼ feet, is as
.4433 millimeters/foot × 26¼ feet =